First off, this program is absolutely amazing and I absolutely love it! I've only had it for a day, but I already feel like it is helping me focus better on what I'm writing!
One issue I'm noting is that the Clear Crystal theme uses white text, but it makes it difficult to read when the background also has light colors. I know you've mentioned that you don't intend to add any font formatting, but I wanted to know if that included font colors? Even if it is just switching between black and white, I think it would make it a lot easier to read the text in those situations. (I have a couple of examples below). As an alternative, you could also have a "Dark" Clear Crystal theme that uses black text.
I also wanted to make a suggestion for a theme. I really enjoy the raindrop noise, so I thought a background of raindrops hitting a window with a foresty background would be really neat.
Thank you again for making this!