DEV: Use CSS custom properties for colors in SCSS

DEV: Use CSS custom properties for colors in SCSS

Prepares the plugin for an upcoming change in core while maintaining backwards compatiblity.

diff --git a/assets/stylesheets/common/graphviz.scss b/assets/stylesheets/common/graphviz.scss
index 2fd46d8..3aad85d 100644
--- a/assets/stylesheets/common/graphviz.scss
+++ b/assets/stylesheets/common/graphviz.scss
@@ -4,14 +4,14 @@ div.graphviz {
   }
   &.is-loading {
     padding: 2em;
-    background: $primary-very-low;
+    background: var(--primary-very-low, $primary-very-low);
   }
 
   .graph-error {
     padding: 2em;
-    background-color: $danger-low;
+    background-color: var(--danger-low, $danger-low);
     margin: 0;
-    color: $danger;
+    color: var(--danger, $danger);
     text-align: center;
   }
 }

GitHub sha: eecd5124