[Admin] Themes management

[e056]

• [x] View all existing themes
• [x] Add new theme
• [x] Disable existing theme -> will this affect existing bookings/activities? or does disabling just mean that subsequent activities created will not have that theme in the dropdown? how will this impact the activity filter on the client side? Does disabling a theme mean that that theme will not be available for filtering in the client page?
• [ ] Search existing themes