I ended up just replacing the entire theme.txt file with that of Processing 3. Or rather, everything before the line full of exclamation points. Because as it turns out, everything after that line contains the colors for highlighted text (such as void turning cyan, float or int in a declaration turning orange, or a string of text turning purple). So I left that as is.
As a result, what I got was an interface looking almost identical to that in Processing 3. With one admittedly very large difference. It turns out that Processing 4 decided to use a slightly different color scheme than Processing 3, which in some cases causes a contrast difference that makes certain text difficult to see. I don’t really know how to fix this. Or I guess I do, I just don’t really feel like it. The best idea I can think of here involves taking screenshots of code from Processing 3, sampling the text pixel by pixel to find the approximate color. Then fiddle around with the theme file until you properly implement all the colors. Which, yeah, that’s why I don’t really feel like doing this. Especially since someone else is bound to come up with a more elegant / less stupid solution to this.