Bug 1966080 - Fix clear icon fill in search-textbox. r=emilio,desktop-theme-reviewers

By setting the -moz-context-properties: fill on the
.textbox-search-clear class, we ensure the the clear icon updates its
fill correctly when switching between light and dark themes.

Differential Revision: https://phabricator.services.mozilla.com/D249135

toolkit/themes/shared/search-textbox.css

@@ -82,6 +82,8 @@ input {
 
 .textbox-search-clear {
   list-style-image: url(resource://content-accessible/close-12.svg);
+  -moz-context-properties: fill;
+  fill: currentColor;
 }
 
 .textbox-search-icon:not([disabled]) {