I'd like to know this too, I've tried to change the font SIZE but can only seem to change the color. Why is it so disorganized in Reaper when it comes to customizing lol some edits are PNG hacks, some are theme tweaks, it's so convoluted to find where all these are ;p
Any guru out there can help? I'm tired of squinting in Reaper, I mean, it's like Kontakt sometimes! Damn Tedious tiny arrows and squinty fonts LOL haha
Sorry, but this is a joke on a 28" 1080p HDMI monitor in 2016 folks: