Add transparency/opacity for queue sidebar (#231)

* add opacity

* add background for song metadata

* Add padding and border radius to opacity elements

* Remove font-weight transition on active lyrics (#233)

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Co-authored-by: jeffvli <jeffvictorli@gmail.com>

src/renderer/themes/default.scss

@@ -8,6 +8,7 @@
     --danger-color: rgb(204, 50, 50);
     --generic-border-color: rgba(50, 50, 50, 30%);
     --main-bg: rgb(18, 18, 18);
+    --main-bg-transparent: 18, 18, 18;
     --main-fg: rgb(225, 225, 225);
     --main-fg-secondary: rgb(150, 150, 150);
     --window-bar-bg: rgb(16, 16, 16);