I am using the Fluida Theme. In order to control an event calendar font size, I added a few css lines. When I upgraded I lost these. Since, I have found the nice feature to add in custom css over-rides under Misc. Will these over-rides persist when I upgrade in the future or will I need to re-enter them.
[In the doing the upgrade, a few settings did not persist – such as static home page and background color for the footer — both easy to fix.]
Yes, the content of the Custom CSS field is retained over updates.
The footer recently received a couple more colours and existing one were rearranged.
The static page setting is not a theme option but under WordPress’ control (the theme can indeed override it with its landing page feature, but this can be disabled).