dev: used var for consistency

Theme/ElegantFin-theme-nightly.css

@@ -914,7 +914,7 @@ html {
 
 .itemProgressBar:not(.backgroundProgress, .playbackProgress, .transcodingProgress) {
     backdrop-filter: var(--blurLarge);
-    background-color: rgba(169, 169, 169, 0.3) !important;
+    background-color: var(--osdSeekBarBufferedColorAlpha) !important;
     height: 0.35em;
     overflow: hidden;
     border-radius: var(--largeRadius);