fix(css/chrome): Use standard property for scrollbar (#816)

This was recently standardized and is currently implemented in Firefox Nightly.

Author: sorin-davidoi

src/theme/css/chrome.css

@@ -8,7 +8,9 @@
 ::-webkit-scrollbar-thumb {
     background: var(--scrollbar);
 }
-
+html {
+    scrollbar-color: var(--scrollbar) var(--bg);
+}
 #searchresults a,
 .content a:link,
 a:visited,