CKEditor "Formatting Styles" drop-down is not picking up styles from the current installed theme editor styles when editing a content block within Stacks.

I can confirm this.
Hmm, technically this is because there is no active theme when you're actually in a stack. It'd be easy enough to use the default site theme, but that's not always what you'd want to do (although it would be in 99.99% of cases)
I just ran across this issue as well. Would it be possible to either force it to the default site theme as you described, or to include the styles from all of the site's themes?
Yeah, thinking more about this I think we should just use the default site theme in stacks.
@aembler
I believe this issue can be closed. https://github.com/concrete5/concrete5/pull/6461
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Yeah, thinking more about this I think we should just use the default site theme in stacks.