Out of interest what padding are you referring to?
I would argue the main areas are pretty much covered.
- Selection of inbuilt themes
- Addition of theme via plugin (like macOS theme)
- Addition of themes via CSS
- UI modification for advanced users by CSS.
I wouldn't say that eliminating UI padding by modifying the application CSS is a standard user practice, many apps don't offer anywhere near that level of granularity outside of advanced config such as css.