Don't override the default selection colours (#2479)

Fixes #2139

NEWS.md

@@ -1,5 +1,6 @@
 # pkgdown (development version)
 
+* pkgdown no longer overrides the default selection colours. This improves accessibility for users who have set their own colours in their browser settings (#2139, @glin).
 * `build_article()` now escapes html characters in the title (#2286).
 * `build_article()` no longer generates the wrong source link when you build your site outside of the root directory (#2172).
 * `build_reference()` matches usage for S3 and S4 methods to the style used by R 4.0.0 and later (#2187).

inst/BS5/assets/pkgdown.scss

@@ -207,10 +207,6 @@ body > .container .row {
 
 /* General typography ======================================================= */
 
-::selection {
-  background-color: mix($primary, $body-bg, 20%);
-}
-
 // Ensure in-page images don't run outside their container
 main img {
   max-width: 100%;