I think that might be difficult because then the lemmy server would have to serve that custom css to each user every time the website is loaded, incurring unnecessary server load.
the title bar doesnt go from the right to left anymore but only in the center
Some UI elements have been moved in the new version so they aren't "blue buttoned" anymore with your style which I liked:
Settings comparison: 0.17.4 vs 0.18.0
Frontpage as well: 0.18.0
Looks great! I for one am excited to see what we can do with theming; I'd personally love for Lemmy to be just a tad more information dense, like the old.reddit.com was, but these curves edges are excellent for now :)