feat: add brand color system, showtext fonts, and dark theme

- R/colors.R: civilytics_colors named vector (from --cv-* CSS vars on
  civilytics.com), three palettes (main/sequential/diverging),
  scale_color_civilytics() and scale_fill_civilytics() for ggplot2
- R/fonts.R: auto-loads Inter + Libre Franklin from Google Fonts via
  showtext in .onLoad(); exports civilytics_load_fonts() for manual retry
- R/theme.R: wires ggplot2 4.0 ink/paper/accent params into theme_grey()
  base, adds title_family + strip_color params; new theme_civilytics_dark()
  using navy_dark background and warm off-white text
- R/logo.R: make_logo_grob(variant) accepts "light"/"dark" to select the
  white logo for dark-background compositions
- DESCRIPTION: adds showtext, sysfonts; pins ggplot2 >= 4.0.0
- tests/testthat/test_theme.R: full test suite for colors, palettes,
  scales, both theme variants, and logo grob variants

R/logo.R

@@ -121,12 +121,14 @@ has_caption <- function(gg) {
 #' @export
 #'
 #' @examples
+#' \dontrun{
 #' library(ggplot2); library(grid)
 #' tmp_plot <- ggplot(mtcars) + aes(x = hp, y = disp) + geom_point() + theme_civilytics()
 #' tmp_logo <- make_logo_grob()
 #' plot_and_logo <- add_logo(tmp_plot, tmp_logo)
 #' grid.draw(plot_and_logo)
 #' dev.off()
+#' }
 add_logo_ga <- function(plot_list, logo, nrow = 1, widths = NULL, margin_param = NULL) {
   # Change position of logo depending on if plot has a caption
   if (!is.null(margin_param)) {
@@ -157,17 +159,28 @@ add_logo_ga <- function(plot_list, logo, nrow = 1, widths = NULL, margin_param =
 
 #' Get a Civilytics Logo grob
 #'
-#' @return a gg object which contains the logo file stored as a Grob suitable for manipulating in
-#' grid
+#' Returns a ggplot object containing the Civilytics logo as a rasterGrob,
+#' ready to compose with plots via [add_logo()] or [add_logo_ga()].
+#'
+#' @param variant Character. `"light"` (default) uses the standard dark logo
+#'   suitable for light backgrounds. `"dark"` uses the white logo suitable for
+#'   dark backgrounds (pairs with [theme_civilytics_dark()]).
+#'
+#' @return A ggplot object (class `"gg"`) containing the logo grob.
 #' @export
-#' @import ggplot2
+#' @importFrom ggplot2 ggplot theme_void annotation_custom
 #' @examples
-#' logo <- make_logo_grob()
-#' class(logo) # gg
-make_logo_grob <- function() {
-  ggplot() +
-    theme_void() +
-    annotation_custom(get_png(system.file("img", "civilytics_logo.png",
-                                          package = "civilytics")), xmin = 0.7, xmax = 1)
+#' logo       <- make_logo_grob()           # light background
+#' logo_dark  <- make_logo_grob("dark")     # dark background
+#' class(logo)  # "gg" "ggplot"
+make_logo_grob <- function(variant = c("light", "dark")) {
+  variant  <- match.arg(variant)
+  img_file <- if (variant == "dark") "civilytics_logo_white.png" else "civilytics_logo.png"
+  ggplot2::ggplot() +
+    ggplot2::theme_void() +
+    ggplot2::annotation_custom(
+      get_png(system.file("img", img_file, package = "civilytics")),
+      xmin = 0.7, xmax = 1
+    )
 }