Shiny Bslib Theming Agent Skill

Theming Shiny Apps with bslib

Customize Shiny app appearance using bslib's Bootstrap 5 theming system. From quick Bootswatch themes to advanced Sass customization and dynamic color mode switching.

Quick Start

"shiny" preset (recommended starting point):

page_sidebar(
  theme = bs_theme(),  # "shiny" preset by default — polished, not plain Bootstrap
  ...
)

Bootswatch theme (for a different visual style):

page_sidebar(
  theme = bs_theme(preset = "zephyr"),  # or "cosmo", "minty", "darkly", etc.
  ...
)

Custom colors and fonts:

page_sidebar(
  theme = bs_theme(
    version = 5,
    bg = "#FFFFFF",
    fg = "#333333",
    primary = "#2c3e50",
    base_font = font_google("Lato"),
    heading_font = font_google("Montserrat")
  ),
  ...
)

Auto-brand from _brand.yml:

If a _brand.yml file exists in your app or project directory, bs_theme() automatically discovers and applies it. No code changes needed. Requires the brand.yml R package.

bs_theme(brand = FALSE)    # Disable auto-discovery
bs_theme(brand = TRUE)     # Require _brand.yml (error if not found)
bs_theme(brand = "path/to/brand.yml")  # Explicit path

Theming Workflow

• Start with the "shiny" preset (default) or a Bootswatch theme close to your desired look
• Customize main colors (bg, fg, primary)
• Adjust fonts with font_google() or other font helpers
• Fine-tune with Bootstrap Sass variables via ... or bs_add_variables()
• Add custom Sass rules with bs_add_rules() if needed
• Enable thematic::thematic_shiny() so plots match the theme
• Use bs_themer() during development for interactive preview

Example:

theme <- bs_theme(preset = "minty") |>
  bs_theme_update(
    primary = "#1a9a7f",
    base_font = font_google("Lato")
  ) |>
  bs_add_rules("
    .card { box-shadow: 0 2px 8px rgba(0,0,0,0.1); }
  ")

bs_theme()

Central function for creating Bootstrap themes. Returns a sass::sass_bundle() object.

bs_theme(
  version = version_default(),
  preset = NULL,        # "shiny" (default for BS5+), "bootstrap", or Bootswatch name
  ...,                  # Bootstrap Sass variable overrides
  brand = NULL,         # brand.yml: NULL (auto), TRUE (require), FALSE (disable), or path
  bg = NULL, fg = NULL,
  primary = NULL, secondary = NULL,
  success = NULL, info = NULL, warning = NULL, danger = NULL,
  base_font = NULL, code_font = NULL, heading_font = NULL,
  font_scale = NULL,    # Scalar multiplier for base font size (e.g., 1.5 = 150%)
  bootswatch = NULL     # Alias for preset
)

Use bs_theme_update(theme, ...) to modify an existing theme. Use is_bs_theme(x) to test if an object is a theme.

Presets and Bootswatch

The "shiny" preset (recommended): bs_theme() defaults to preset = "shiny" for Bootstrap 5+. This is a polished, purpose-built theme designed specifically for Shiny apps — it is not plain Bootstrap. It provides professional styling with well-chosen defaults for cards, sidebars, value boxes, and other bslib components. Start here and customize with colors and fonts before reaching for a Bootswatch theme.

Vanilla Bootstrap: Use preset = "bootstrap" to remove the "shiny" preset and get unmodified Bootstrap 5 styling.

Built-in presets: builtin_themes() lists bslib's own presets.

Bootswatch themes: bootswatch_themes() lists all available Bootswatch themes. Choose one that fits the app's purpose and audience — don't apply one by default.

Popular options: "zephyr" (light, modern), "cosmo" (clean), "minty" (fresh green), "flatly" (flat design), "litera" (crisp), "darkly" (dark), "cyborg" (dark), "simplex" (minimalist), "sketchy" (hand-drawn).

Main Colors

The most influential colors — changing these affects hundreds of CSS rules via variable cascading:

| Parameter | Description |

|---|---|

| bg | Background color |

| fg | Foreground (text) color |

| primary | Primary brand color (links, nav active states, input focus) |

| secondary | Default for action buttons |

| success | Positive/success states (typically green) |

| info | Informational content (typically blue-green) |

| warning | Warnings (typically yellow) |

| danger | Errors/destructive actions (typically red) |

bs_theme(
  bg = "#202123", fg = "#B8BCC2",
  primary = "#EA80FC", secondary = "#48DAC6"
)

Color tips:

• bg/fg: similar hue, large luminance difference (ensure contrast for readability)
• primary: contrasts with both bg and fg; used for hyperlinks, navigation, input focus
• Colors can be any format htmltools::parseCssColors() understands

Typography

Three font arguments: base_font, heading_font, code_font. Use font_scale to uniformly scale all font sizes (e.g., 1.5 for 150%).

Each argument accepts a single font, a font_collection(), or a character vector of font names.

font_google()

Downloads and caches Google Fonts locally (local = TRUE by default). Internet needed only on first download.

bs_theme(
  base_font = font_google("Roboto"),
  heading_font = font_google("Montserrat"),
  code_font = font_google("Fira Code")
)

With variable weights: font_google("Crimson Pro", wght = "200..900")

With specific weights: font_google("Raleway", wght = c(300, 400, 700))

Recommend fallbacks to avoid Flash of Invisible Text (FOIT) on slow connections:

bs_theme(
  base_font = font_collection(
    font_google("Lato", local = FALSE),
    "Helvetica Neue", "Arial", "sans-serif"
  )
)

Font pairing resource: fontpair.co

font_link()

CSS web font interface for custom font URLs:

font_link("Crimson Pro",
  href = "https://fonts.googleapis.com/css2?family=Crimson+Pro:[email protected]")

font_face()

For locally hosted font files with full @font-face control:

font_face(
  family = "Crimson Pro",
  style = "normal",
  weight = "200 900",
  src = "url(fonts/crimson-pro.woff2) format('woff2')"
)

font_collection()

Combine multiple fonts with fallback order:

font_collection(font_google("Lato"), "Helvetica Neue", "Arial", "sans-serif")

Low-Level Theming Functions

For customizations beyond bs_theme()'s named parameters. These work directly with Bootstrap's Sass layers.

bs_add_variables()

Add or override Bootstrap Sass variable defaults:

theme <- bs_add_variables(
  bs_theme(preset = "sketchy", primary = "orange"),
  "body-bg" = "#EEEEEE",
  "font-family-base" = "monospace",
  "font-size-base" = "1.4rem",
  "btn-padding-y" = ".16rem"
)

The .where parameter controls placement in the Sass compilation order:

| .where | When to use |

|---|---|

| "defaults" (default) | Set variable defaults with !default flag. Placed before Bootstrap's own defaults. |

| "declarations" | Reference other Bootstrap variables (e.g., $secondary). Placed after Bootstrap's defaults. |

| "rules" | Placed after all rules. Rarely needed. |

Referencing Bootstrap variables:

# This fails in bs_theme() because $secondary isn't defined yet:
# bs_theme("progress-bar-bg" = "$secondary")

# Use bs_add_variables with .where = "declarations" instead:
bs_theme() |>
  bs_add_variables("progress-bar-bg" = "$secondary", .where = "declarations")

bs_add_rules()

Add custom Sass/CSS rules that can reference Bootstrap variables and mixins:

theme <- bs_theme(primary = "#007bff") |>
  bs_add_rules("
    .custom-card {
      background: mix($bg, $primary, 95%);
      border: 1px solid $primary;
      padding: $spacer;

      @include media-breakpoint-up(md) {
        padding: $spacer * 2;
      }
    }
  ")

From external file: bs_add_rules(sass::sass_file("www/custom.scss"))

Available Sass functions: lighten(), darken(), mix(), rgba(), color-contrast().

Available Bootstrap mixins: @include media-breakpoint-up(), @include box-shadow(), @include border-radius().

bs_add_functions() and bs_add_mixins()

Add custom Sass functions or mixins to the theme bundle:

theme |>
  bs_add_functions("@function my-tint($color) { @return mix(white, $color, 20%); }") |>
  bs_add_rules(".highlight { background: my-tint($primary); }")

bs_bundle()

Append sass::sass_bundle() objects to a theme (for packaging reusable theme extensions):

my_extension <- sass::sass_layer(
  defaults = list("my-var" = "red !default"),
  rules = ".my-class { color: $my-var; }"
)
theme <- bs_theme() |> bs_bundle(my_extension)

Bootstrap Sass Variables

Pass any Bootstrap 5 Sass variable through bs_theme(...) or bs_add_variables().

Finding variable names: https://rstudio.github.io/bslib/articles/bs5-variables/

Common variables:

bs_theme(
  "border-radius" = "0.5rem",
  "card-border-radius" = "1rem",
  "card-bg" = "lighten($bg, 5%)",
  "navbar-bg" = "$primary",
  "link-color" = "$primary",
  "font-size-base" = "1rem",
  "spacer" = "1rem",
  "btn-padding-y" = ".5rem",
  "btn-padding-x" = "1rem",
  "input-border-color" = "#dee2e6"
)

Values can be Sass expressions referencing variables, functions, and math.

Bootstrap CSS Custom Properties

See sass-and-css-variables.md for details on:

• How Sass variables compile into --bs-* CSS custom properties
• Runtime vs compile-time variable layers
• How Bootstrap 5.3 color modes use CSS variable overrides
• Per-element theming with data-bs-theme
• CSS utility classes for one-off styling

Dark Mode and Color Modes

See dark-mode.md for details on:

• Bootstrap 5.3's client-side color mode system (data-bs-theme attribute)
• input_dark_mode() and toggle_dark_mode() for user-controlled switching
• Server-side theme switching with session$setCurrentTheme()
• Writing custom Sass that works across light/dark modes
• Component compatibility (what responds to theming, what doesn't)

Theming R Plots

bs_theme() only affects CSS. R plot output (rendered server-side as images) won't auto-match. Use the thematic package:

library(thematic)
thematic_shiny(font = "auto")  # Call before shinyApp()
shinyApp(ui, server)

• Works with base R, ggplot2, and lattice
• Translates CSS colors into R plotting defaults
• font = "auto" also matches fonts from bs_theme()
• Complements bs_themer() for real-time preview

Set global ggplot2 theme for further consistency:

library(ggplot2)
theme_set(theme_minimal())

Dashboard Background Styling

The bslib-page-dashboard CSS class adds a light gray background behind the main content area, giving dashboard-style apps a polished look where cards stand out against the background. This is a theming detail — it doesn't change layout behavior, only the visual treatment.

For page_sidebar() dashboards:

page_sidebar(
  class = "bslib-page-dashboard",
  title = "My Dashboard",
  sidebar = sidebar(...),
  ...
)

For page_navbar() with dashboard-focused pages:

Apply the class to individual nav_panel() containers (not page_navbar() itself) so only dashboard-oriented pages get the gray background:

page_navbar(
  title = "Analytics",
  nav_panel("Dashboard", class = "bslib-page-dashboard",
    layout_column_wrap(...)
  ),
  nav_panel("Report",
    # No dashboard class — standard white background for prose/reports
    ...
  )
)

Interactive Theming Tools

bs_theme_preview()

Standalone demo app for previewing a theme with many example UI components:

bslib::bs_theme_preview()                        # Default theme
bslib::bs_theme_preview(bs_theme(preset = "darkly"))  # Custom theme

Includes the theming UI by default (with_themer = TRUE).

run_with_themer()

Run an existing Shiny app with the theme editor overlay (instead of shiny::runApp()):

run_with_themer(shinyApp(ui, server))
run_with_themer("path/to/app")

bs_themer()

Add the theme editor to your own app's server function:

server <- function(input, output, session) {
  bs_themer()  # Add during development, remove for production
  # ...
}

All three tools print the resulting bs_theme() code to the R console for easy copy-paste. Limitations: Bootstrap 5+ only, Shiny apps and runtime: shiny R Markdown only, doesn't affect 3rd-party widgets that don't use bs_dependency_defer().

Theme Inspection

Retrieve computed Sass variable values:

vars <- c("body-bg", "body-color", "primary", "border-radius")
bs_get_variables(bs_theme(), varnames = vars)
bs_get_variables(bs_theme(preset = "darkly"), varnames = vars)

Check contrast (for accessibility):

bs_get_contrast(bs_theme(), c("primary", "dark", "light"))

Aim for WCAG AA compliance: 4.5:1 for normal text, 3:1 for large text.

Best Practices

• Prefer bs_theme() over custom CSS -- variables cascade to all related components automatically
• Pin Bootstrap version: bs_theme(version = 5) prevents breakage if defaults change
• Use fallback fonts with font_collection() to avoid FOIT on slow connections
• Test across components: inputs, buttons, cards, navs, plots, tables, modals, toasts, mobile
• Check accessibility with bs_get_contrast() and browser dev tools
• Use CSS utility classes for one-off styling instead of custom CSS (see sass-and-css-variables.md)
• Organize complex themes in a separate theme.R:

# theme.R
app_theme <- function() {
  bs_theme(
    version = 5,
    primary = "#2c3e50",
    base_font = font_google("Lato"),
    heading_font = font_google("Montserrat", wght = c(400, 700))
  ) |>
    bs_add_rules(sass::sass_file("www/custom.scss"))
}

Reference Files

• sass-and-css-variables.md -- Bootstrap's two-layer variable system, CSS custom properties, utility classes
• dark-mode.md -- Color modes, dark mode, dynamic theming, component compatibility