Hello Kalpesh Katyare,
You can add customized theme as a zip file inside your project directory inside the theme folder and then select the particular theme zip file in the Project settings inside mendix studio pro 8.8.1.
Another approach to do this was explained here → https://docs.mendix.com/refguide/project-settings#6-theme-tab
Hope this answer helps you to figure out your requirement, Thanks!
Thanks Ananthi A.