fix gitgraph commit hashes in dark theme (#10046)

because the CSS was lazy-loaded the rules in arc-green did not win.
included the css file in the main bundle to fix. the black dots can not
be fixed via CSS because they are drawn in a `<canvas>` element
unfortunately.

web_src/less/index.less

@@ -14,3 +14,4 @@
 @import "_explore";
 @import "_review";
 @import "~highlight.js/styles/github.css";
+@import "../css/gitGraph.css";