We started of with Communities in Connections allowing custom themes, but after applying branding, we wanted to disable the custom themes.
The custom themes in Connections Communities 5.0 don’t all use the next gen theme yet, so with custom branding, some things look really ugly.

Now when you disable custom themes, but this does not affect existing communities. You can ask your community owners to please reset their themes to default, but you can also do it for them. Unfortunately, doing this through the API does not work (you can set everything through the API, but in this version 5.0 CR2, setting the theme appears to be broken - we’ve opened a PMR), so we’re left with editing the database directly.
You need to update multiple databases/schema’s, not just the Community schema ! Here’s what to do :

Of course, don’t edit the Connections databases directly, ever …. so use these at your own risk :-)

Communities:

UPDATE "SNCOMM"."COMMUNITY" SET COMMUNITY_THEME = 'default'

Wikis:

UPDATE "WIKIS"."LIBRARY" SET THEME_NAME = 'default'

Blogs:

UPDATE "BLOGS"."WEBSITEASSOC" SET ASSOCTHEME = 'default'

Forums:

UPDATE "FORUM"."DF_MEMBERPROFILE" SET THEMEID = 'default' WHERE THEMEID is not null

Activities:

UPDATE "ACTIVITIES"."OA_MEMBERPROFILE" SET THEMEID = 'default' WHERE THEMEID is not null