Unfortunately it’s different, you can’t reuse your obsidian.css in here. However if you dedicate yourself to it it’s not that much harder to customize this css.
Just for reference I adapted my Obsidian Theme Wasp with similar colors into my Logseq Theme Cobra, I do know css, but I’m no expert.
What I’d say is if you managed to create an Obsidian theme, you can definitely create a logseq theme
I recommend looking at masters of css like @cannibalox 's theme Darker hpx for a great css example, which I based my logseq theme on.