Enhanced Theming enables users to customize SharePoint themes further by selecting different colors and fonts. Enhanced theme is actually a hidden feature (GUID: 068bc832-4951-11dc-8314-0800200c9a66), stapled with many of the site templates by the BaseSiteStapling feature. It doesn’t come up with “Blank Site” template, BTW.
If you want to restrict users from customizing themes further, you can disable enhanced theming feature. Here is how:
Disable-SPFeature -identity enhancedtheming -url https://sharepoint.company.com
Before Disabling Enhanced Theme: