How do I change the top right-hand logo in Mastodon? (v4.2.x)

A reddit user explains how to do it with CSS here:

https://www.reddit.com/r/Mastodon/comments/1du3d20/comment/ldimccc/?context=3

I have used this on various versions of Mastodon in the v4.2.x branch, although I think that can be done more easily in v4.3, but I have not upgraded yet.