Many themes store option settings in the WordPress database. When you reactivate an older theme, if the options in the theme menu are already filled in, that is most likely the case. You would then need to press “Save” (or the equivalent) in the theme menu to regenerate the required CSS and Control files for the browser from those options stored in the database.
After you make the changes, make sure that you clear the cache and “hard” refresh the page in your browser to be able to see the changes right away. Also, don’t forget if you have any caching WordPress plugins installed, or use a CDN (like CloudFlare) to also either clear their cache or disabled them for the time being altogether. If you’re not sure how to “hard” refresh, this site has some great tutorials on how to clear your cache: How to Clear Your Browser’s Cache Instructions.
Also check with your host and make sure that if they use a caching system above your site, that that is being cleared, too. For example: https://support.godaddy.com/help/article/8057/clearing-the-server-cache-using-website-accelerator
