wyrd
Asking them some genuine non-confrontational questions about their feelings or possible solutions to the situation usually works fine for me. I feel like people often vent about situations where they feel kinda helpless. Questions from you could help them not only to feel seen/heard, but also to switch to a more “proactive” state of mind where the source of venting could start looking like a problem with possible solutions.
Other times people just want to vent for the sake of venting and want no input from you whatsoever. Just let them.
I personally belong to the dark theme cult, but this spring we had to rent a flat for a while where ambient light conditions were just horrible. For the first time in my life I just HAD to use light themes everywhere because dark themes would just make my eyes hurt after a while. So maybe people who prefer light themes just spend most of their time somewhere with dim uneven lights?