Add medium grey theme ( or another eye saving theme )

### Is there an existing issue for this?

- [x] I have searched the existing issues and checked the recent builds/commits

### What problem are you experiencing?

The light theme is a bit too bright for me.
And all other themes are a bit too dark.


### When does this problem occur?

When working for a long time, my eyes get tired, if the UI is very dark or very bright. 

### How often do you encounter this problem?

Multiple times per day

### How much does this problem affect your workflow?

Causes moderate inconvenience

### Current workarounds

We can create a custom theme.
But that's a bit tricky for regular users.

### Ideas for solutions (Optional)

Blender has some nice grey themes.
Could it be possible to have something similar ?

https://extensions.blender.org/themes/theme-xsi/


https://extensions.blender.org/themes/neutral-grey/


### Additional context

_No response_