[Feature]: Add options for higher contrast/larger font size, especially with usernames

[NovemberOrWhatever]

Precondition

• I understood that this template is only for feature requests and not for bug reports

Cross-checked related issues?

• I have cross-checked this overview Overview: Tasks and Issues #322 as well as filtered for related labels https://github.com/monal-im/Monal/labels

Describe your feature

I personally find the contrast between the default background and the font color for usernames insufficient for figuring out who's who at a glance and would really like something higher-contrast, maybe making the text lighter and the background darker, or adding settings for font size and color. General UI scale options would be nice as well, but I understand that would be harder. Thanks.