As you know, Tribe's native Theme customization settings currently do not provide gradient color options.
However, If you happen to know how to code, you can do some advanced customizations by going to Admin panel> theme> advanced settings> from there, you can add custom* CSS codes.
Customization is interesting and we know that most Community Builders would like to show their creativity by personalizing the look of their platform. However, if you are not comfortable with coding, we do not recommend making lots of changes via CSS, as it might overwrite other components in your community and put you in trouble.
Having said that, Tribe customers are sharing their CSS codes with each other here3 in our support community. I tagged your question with this Topic so that it surfaces to the CSS experts in this community - they may be able to help out!
Just to add more context to my answer, please be advised that Tribe Team does not provide services like checking customized codes. You can learn more about the level of services we provide here1.
Let me know if you found this helpful!