Hi @Christiane - welcome to the community!
You can definitely use Adobe fonts for email templates as well as the Klaviyo Signup Forms.
For the Klaviyo Signup Forms, you have to first upload it into your Brand Library (Sidebar → Brand Library → Fonts). Then, in your Signup Forms, you can select that Font in the Block styles or text editor in the Klaviyo Signup Form editor. Specific step-by-step instructions are here:
For Email Templates, it depends if you’re using the Classic Template Editor, or the New Template Editor. If you’re not sure which one you’re using, this article explains the difference:
For the Classic Editor, you can follow the instructions here:
If you have been using the New Template Editor, you can follow the instructions here:
Hope this helps!