When using the Litehtml-viewer plugin, the text is rendered as black, even in dark theme, making it difficult to read the email. The litehtml email background should be rendered as a light color (maybe light grey for a dark theme and white for a light theme) or the litehtml email text should be made lighter to be readable against the dark background. This should be done automatically, but I also saw no way in the claws-mail settings or the litehtml plugin settings to manually change these colors, which would also be useful (only for html view mode). I don't know if this is a claws-mail or litehtml problem (or something else).
I had the same problem and switched to claws-mail-fancy-plugin. There you have the option to specify a custom stylesheet, here's an example: html,body,table { font-family: LiberationSans; font-size: 13px; background: #2d2d2d; color: white; } a { color: #00e38f; text-decoration: none; }