When utilising dark desktop themes it is impossible to read the text from HTML emails which fail to set a background with dark text on dark backgrounds. A possible workaround is to use per-application theming and choose a lighter theme for KMail, which has been discussed and implemented in KColorSchemeManager I think (see bug #279592)?
I can confirm that HTML email often gets unreadable when using Breeze Dark. How about a simpler approach: whenever an email is shown as HTML (regardless of automatic setting or manually clicking the HTML Message button), the setting "Do not change the color of the original email" is enabled automatically. HTML is extensively used to portray style (if not the main reason to use it to begin with). This often includes things like background colors, and it seems undesirable to nullify such things when reading email, especially when it's not possible to ensure readability and proper contrast. Doing this will not require KMail itself to have a light theme in order for HTML email to be readable.
*** Bug 436971 has been marked as a duplicate of this bug. ***
This is still an issue for me on Linux Mint 21.3 Cinnamon with the Kontact 23.08.4 flatpak.
I confirm in Plasma 6 24.02