The dark (green) theme (and probably the other dark theme) has an ugly white border around some elements. The ones I found with this issue are the text editor and the sidebar on profiles.
Also the bar at the top (the one with the "Forums", "Leaderboard" and "Chatbox" buttons) is a really dark green which doesn't stand out from the dark grey background.
This is the case for the main page of the forums too.
Also, the heart reaction is blue, which does not fit well with the dark (green) theme. It also has a white border.
Also, if possible, could you please fix the low contrast between the quote box and text, and the spoiler box and text in the text editor? (The low contrast only occurs in the text editor NOT once it has been posted. It's really hard to read what you're typing in the spoiler box)
That's all the problems I've found so far, but I'll keep you informed if I find anymore.
(Also, I'm liking the reactions. Is it possible for you to add more or are they fixed?)