Poor contrast of sender names on dark theme

- Operating System (Linux/Mac/Windows/iOS/Android): all
- Delta Chat Version: v2.35.0, c64d6edef0a50d3e07288eaaad90d28d47405f6f
- Expected behavior: Contrast is good
- Actual behavior: Contrast is bad
- Steps to reproduce the problem:
1. Set color theme to dark.
1. Create a group.
2. Receive a message in that group.
- Screenshots:

The colors are generated in core. The situation got a little worse when the colors were changed recently (like a few months back). But either way I am not sure whether there _is_ a way to always use the same colors between dark and light themes. Would simply lightening the colors make them look good on both light and dark themes?
Maybe we'd need some manual front-end site lightening or darkening. Or make Core do that, but that would be very clumsy.
Another possible solution is to always have light background for these colors.

Telegram also utilizes colored names, maybe we could look up how they solve the issue.

Android and iOS are also affected by this issue, but a little less so, because in the dark theme the message bubble is completely black.

https://github.com/deltachat/deltachat-desktop/pull/4259#issuecomment-3517842220