fix: loading indicator should support dark mode

ui/core/app/styles/app.scss

@@ -66,7 +66,7 @@
   &.loading {
     &::after {
       animation: gradient 1s linear infinite;
-      background: repeating-linear-gradient(to right, var(--white), var(--white), var(--info), var(--white), var(--white));
+      background: repeating-linear-gradient(to right, var(--subtle), var(--subtle), var(--info), var(--subtle), var(--subtle));
       background-position: 0 100%;
       background-size: 200% auto;
       content: ' ';