Hi @Allaark,
Last time I saw an issue like this, a theme called Retrodragon had instructions for custom font installation. If the instructions weren’t followed, code at the top of the theme.xml file that set a fallback font would cause a quiet reset.
Newer versions of Aseprite than 1.2.40, in the beta branch, displayed an error message. This might give you a better indicator of the problem with the theme you’re using.
I would go back to the theme’s download website. If the site provided no troubleshooting instructions, then I’d contact the theme’s creator. Unless someone reports an issue, it’s hard for theme creators to know their theme may be obsolete due to Aseprite updates.
In general, there’s also an extension that lets you customize your theme with greater ease.