Someone reached out to me to get more details, as this is still emerging I don't have a lot more details, but I'll do some thinking aloud so at least you have an idea of what's in my head space:
These are the key targets:
1. Clean up of existing Plone themes - many of them need attention, pixels out of place etc...
2. New themes - minimum of one really, really, really good theme (maybe with two or three variations)
3. Contribute to the general theming documentation
4. Tools - I would like to see some improvements to the theming tools used for Plone (Things like plone-themeupload and some of the TTW tools.).
If we're trimming down the scope, 3 would be my "nice-to-have". It could be just reporting any bugs as we work.
To make this happen we'll need to begin some of the work before the sprint (moodboards, creative briefs etc...) possibly 2 months in advance.