[FEATURE] Switch / invert header image depending on theme

[PinkSerenity]

Describe the feature

Set two header images for dark mode and light mode respectively. Alternatively, set one and invert it for the other mode.

Why would it be useful to add?

Line art drawings usually use a single color for the lines. On bright backgrounds they are usually black or very dark, which might work for light mode, but for dark mode you'd need to invert them. Some other images with transparency might also profit from it.

Example(s)

Have some pictures of my kitty!


I know, dark mode isn't optimized yet.

Additional context

This would actually be nice for a lot of things like custom colors for the header clock.