KDE software has a lot of different visual styles for sidebars, many ably illustrated in T11093: Improve Consistency across the Board:
Let's unify these to have the following characteristics:
- White background (with default breeze color scheme; obviously it should follow the color scheme
- Full width and height; top touches titlebar/tools area, bottom touches bottom window edge or status bar, left side touches left window edge, etc.
- Single-pixel vertical line separates the sidebar from the content area on the right
- When icons are large, they should be colorful: T10165: Large category icons should all be colorful