feat(admin): refine theme settings UI

assets/admin/src/components/PageHeader.vue

@@ -39,7 +39,7 @@ defineProps({
 
 .aksrt-page-header__title {
   margin: 0 0 6px;
-  color: #1a1a2e;
+  color: var(--aksrt-color-text);
   font-size: 22px;
   font-weight: 700;
   letter-spacing: -0.02em;
@@ -48,7 +48,7 @@ defineProps({
 
 .aksrt-page-header__desc {
   margin: 0;
-  color: #7a7f8a;
+  color: var(--aksrt-color-muted);
   font-size: 13px;
   line-height: 1.65;
 }