Paletteable and custom reading page colors

SOLVED!

Hello,

I recently switched to Paletteable. I enabled custom reading page colours, but I'd like the custom colours to form a slightly thicker border around the userpic, particularly the "background" colour. (It's hard for me to distinguish some of my colours when the lines are this thin, particularly since I always use black as the "foreground" colour for individual users.)

I've tried to figure out on my own what I should do, but I don't even know where to start.

Thanks in advance!