[FEATURE]: Separate theming of sidebar background color

[jesseleite]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Right now, the sidebar's background color is tied to backgroundPanel, which is the same background color used by chat messages.

I'm looking to make the sidebar's background the same color as the rest of my background, without affecting chat message panels.

Would be rad if we could theme the sidebar with a different background than backgroundPanel used by chat messages 🙏

[github-actions[bot]]

This issue might be a duplicate of or related to existing theming issues. Please check:

• #5088: Theming: Unable to change text input placeholder color - Another theming customization request for colors that can't be changed
• #5184: Confusing colouration of working directory in sidebar - Related to sidebar color theming

Feel free to ignore if your specific use case differs from these.

[ariane-emory]

I'd rather not do anything that would break the existing themes, so, to be safe, it should probably fall back to the existing backgroundPanel property if this new sidebarBackgroundPanel theme property is not present in the activelk theme.