Add missing CSS variables in GUI test for custom-theme.css

Author: GuillaumeGomez

tests/rustdoc-gui/src/theme_css/custom-theme.css

@@ -23,6 +23,8 @@
 --copy-path-button-color:#999;
 --copy-path-img-filter:invert(50%);
 --copy-path-img-hover-filter:invert(35%);
+--code-example-button-color:#7f7f7f;
+--code-example-button-hover-color:#a5a5a5;
 --codeblock-error-hover-color:rgb(255,0,0);
 --codeblock-error-color:rgba(255,0,0,.5);
 --codeblock-ignore-hover-color:rgb(255,142,0);