Accessibility: Color-blind themes for UI

### Your use case

#### What would you like to do?
Add options in Preferences to change certain colors in the UI, to help people with color blindness

#### Why would you like to do it?
I'm colorblind (deuteranopia - green/red), and sometimes it's hard for me to make out whether an user is online (this is an issue that bothers me the most)

#### How would you like to achieve it?

### Have you considered any alternatives?

_No response_

### Additional context

_No response_

No files or tests are named. Start by locating the Preferences UI and the code that displays online status, then determine which UI colors need alternatives for deuteranopia; done would require an agreed design and accessible color options that make user presence distinguishable.

Written by the indexing model from the issue text.