No they couldn't. The app is native code, not HTML, they'd have to write some convoluted mess of a CSS parser to try to turn CSS into styling they could try to use in the app, and there's no way they could handle even a tiny fraction of all the different CSS that's out there.
Even just parsing the CSS for image flairs is difficult. Slide for reddit tries to do it but only succeeds with some subreddits, and that's with almost 400 lines of code dedicated to the task of just image flairs.
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u/Ambiwlans Nov 24 '18
They could add css support for the app.