One well known way is to make a detailed document about the type of design you want v0 to follow and upload it in the sources section, found under Knowledge tab accessible from Settings cog in the top right corner of chat page:
Another way is to follow this guide - Best practices/Suggestions for "Knowledge" instructions , you can include instructions like “Keep the designs, fonts, and element styles consistent across all pages” or something along those lines that best fit your needs!