Minor issue with coloring username in ProfilePreview

[kernelkind]

https://github.com/damus-io/notedeck/blob/69054d71ca020ad74cc5d99d7f3064e0f2a5f5db/src/ui/profile/preview.rs#L98

I don't think the color should be set like this because it will not change when the user changes their theme. The color should be set using some ui.visuals().... I didn't make any colors in colors.rs public to discourage this. @jb55

I'm aware though that Visuals is a bit limiting, we may need to think of a custom solution in the future. Not high priority