Well, this is a nice quality-of-life improvement: the plugin Eclipse Color Themes, which (so far) works just fine in Designer (8.5.3). Having to go through every single editor type and manually pick each color for each code element every time I wanted to change a color theme was always a huge annoyance, particularly compared to other editors. Fortunately, that plugin handles it pretty well, though it unsurprisingly doesn't support LotusScript, so that's still manual. I installed it without problem, found its settings in General\Appearance\Color Themes, installed a port of my preferred theme, and saved myself tons of hassle.