Quick Play does not use custom scroll bar

Relates to MCL-23663.

The Bug:

In every other tab, the scroll bar on the side of the window is a custom gray color. But in Quick Play, the scroll wheel uses the default scroll bar in this case.

Steps to Reproduce:

• Open the Launcher in the version stated and enable experimental features.
• Click on the Minecraft: Java Edition tab and navigate to Quick Play.
• Click the install icon to expand the list of available icons.
• Scroll down in this window and observe the scroll bar on the right.

→  Notice that it's not the custom gray color, but uses the default scroll bar.

Observed Behavior:

The Quick Play scroll bar on the side of the window is not a custom gray color.

Expected Behavior:

The Quick Play scroll bar on the side of the window uses the custom gray color instead of the default scroll bar.