If you run the NimbusNote interface (Light Theme) through an accessibility scanner, it fails pretty badly. Screenshot below. It needs more contrast. This is one thing Evernote gets absolutely right, while other note apps try to be pretty.

This isn't just an issue for people with visual impairment. Once humans hit 40, their ability to distinguish colors starts to diminish. It's not an impairment. It's just aging, and it causes eye strain to look at an interface like NimbusNote has. I'm actually debating going back to Evernote and sacrificing features for the good of my eyes.

Black text on white background is ideal. If the sidebar was actually black and the writing panel white, that would help a lot. I design websites, and I always make the main body text either black or white, nothing else. But even sticking with very light and very dark grays would be great.

You may also want to pay closer attention to accessibility because in the US, there have been a lot of ADA lawsuits in recent years against websites and web services that don't meet accessibility standards.