JetBrains Fleet themes
Fleet has four themes available by default, but one of them is named Sync with OS and depends on the OS setting. In practice, that means you can choose Fleet Dark, Fleet Lite, and Dark Purple.
Personally, I like colors like Dark Purple, but it is a little harsh on the eyes. Still, since Fleet's standard themes did not offer many options, I had been using it. After using Fleet for a while, I realized that additional themes can be installed by selecting Plugins from Settings.
There are not that many at the moment, but there are several options, and you can add themes from there. After trying a few, the one I liked was Catppuccin Frappé. I think the colors are easy on the eyes.
It is also possible to make your own original theme file. The method is described in the link below. When I have a bit of time, I feel like trying to make an original theme.