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

1. Start with the "shiny" preset (default) or a Bootswatch theme close to your desired look
2. Customize main colors (bg, fg, primary)
3. Adjust fonts with font_google() or other font helpers
4. Fine-tune with Bootstrap Sass variables via ... or bs_add_variables()
5. Add custom Sass rules with bs_add_rules() if needed
6. Enable thematic::thematic_shiny() so plots match the theme
7. 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:

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:wght@200..900")

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:

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()