Add ::selection to root color vars (#2437)

* add selection selector

* Create quiet-wasps-care.md

---------

Co-authored-by: Cameron Dutro <[email protected]>

Author: langermank

.changeset/quiet-wasps-care.md

@@ -0,0 +1,5 @@
+---
+"@primer/css": patch
+---
+
+Add `::selection` to root color vars

src/support/mixins/color-modes.scss

@@ -1,6 +1,7 @@
 @mixin color-mode-theme($theme-name, $include-root: false) {
   @if $include-root {
     :root,
+    ::selection,
     [data-color-mode="light"][data-light-theme="#{$theme-name}"],
     [data-color-mode="dark"][data-dark-theme="#{$theme-name}"] {
       @content;
@@ -32,6 +33,7 @@
 @mixin color-mode($mode) {
   @if $mode == light {
     :root,
+    ::selection,
     [data-color-mode="light"][data-light-theme*="#{$mode}"],
     [data-color-mode="dark"][data-dark-theme*="#{$mode}"] {
       @content;