define color of minimap mask via stylesheet

Author: haschek

src/extensions/react-flow/minimap/_minimap.scss

@@ -1,3 +1,15 @@
+.react-flow__minimap {
+    border: solid 1px $eccgui-color-separation-divider;
+
+    & > svg {
+        display: block;
+    }
+}
+
+.react-flow__minimap-mask {
+    fill: rgba($eccgui-color-workspace-background, $eccgui-opacity-muted);
+}
+
 .#{$eccgui}-graphviz__minimap__node--default {
     &:not([fill]) {
         fill: $reactflow-node-background-color;