The RStudio scripting environment didn't do this yesterday: I thought my keyboard was substituting blanks for equal signs. It's putting them onscreen in pale pale pastel yellow. Other colors, while better visible, are pale and pastel.
I messed with "theme" stuff in the global settings, but the preview behavior did not jar the production side out of its pastel propensities. Perhaps "theme" is the wrong thing.
However, I think you and others discovering this thread will be most interested in Using RStudio Themes with RStudio. Coincidentally, this guide was updated today.
Of course monitors and projectors vary in their color accuracy, so it's important to check how all this appears in situ. There are a lot of educators and workshop presenters here, and I think we all have that experience of showing up early and checking how our slides and demos look in the classroom.