At the moment we don't do this generally, but we've been dipping our toes into it somewhat in recent years.

One form this takes is that we now allow sub-project teams to do their own fundraising and "keep the money" while KDE e.V. takes an admin fee, turning us sort of into a financial service provider for KDE projects that is cheaper than places like Open Collective. Our pilot team making use of this is Kdenlive, who spend the money e.g. on contracting dev work on the app.

Another is that we once got a large donation earmarked for and on behalf of Calligra from the Handshake folks. That one to some extent highlights the reasons for our reluctance to do this more generally though - with the Calligra community fairly inactive, finding mechanisms to actually use this money has proven quite difficult so far.

Finally, and this one is probably most interesting to you: The KDE community runs a regular election for community-wide goals; during a past election, there was e.g. one elected for UI/UX consistency. At KDE e.V. we take this election as one of the best signals we have on where we are expected to allocate resources, e.g. funding related dev sprints, or contractor hours. So one of the best ways to effectively earmark money toward thematic/larger stuff, such as the UX consistency you mentioned, is to engage there with goal propsals, discussion, offering support, etc.