How can I change color/theme for email, no dark theme

I'm about to go blind/getting too old for the bright white background in all the themes, are there no DARK themes?

When I searched for help, it said that

The default theme is Cobalt Blue - Dark.

however that doesn't seem to exist, I only see 5 bright themes: Blue, Green, Grey, Brown, Lavender, and PINK, and they all SUCK!

Please help!