UI: Change adv audio background color

The color of the disabled sliders would be the same as the background
in the advanced audio dialog.

UI/data/themes/Acri.qss

@@ -1519,3 +1519,8 @@ QDialogButtonBox {
 OBSBasicStats {
     background: palette(dark);
 }
+
+/* Advanced audio dialog */
+OBSBasicAdvAudio #scrollAreaWidgetContents {
+    background: palette(dark);
+}

UI/data/themes/Grey.qss

@@ -1507,3 +1507,8 @@ QDialogButtonBox {
 OBSBasicStats {
     background: palette(dark);
 }
+
+/* Advanced audio dialog */
+OBSBasicAdvAudio #scrollAreaWidgetContents {
+    background: palette(dark);
+}

UI/data/themes/Light.qss

@@ -1513,3 +1513,8 @@ QDialogButtonBox {
 OBSBasicStats {
     background: palette(dark);
 }
+
+/* Advanced audio dialog */
+OBSBasicAdvAudio #scrollAreaWidgetContents {
+    background: palette(dark);
+}

UI/data/themes/Rachni.qss

@@ -1511,3 +1511,8 @@ QDialogButtonBox {
 OBSBasicStats {
     background: palette(dark);
 }
+
+/* Advanced audio dialog */
+OBSBasicAdvAudio #scrollAreaWidgetContents {
+    background: palette(dark);
+}

UI/data/themes/Yami.qss

@@ -1511,3 +1511,8 @@ QDialogButtonBox {
 OBSBasicStats {
     background: palette(dark);
 }
+
+/* Advanced audio dialog */
+OBSBasicAdvAudio #scrollAreaWidgetContents {
+    background: palette(dark);
+}