Hi!
1. Can you enter the custom CSS there again? In case you use any form-fill browser plugins, I suggest turning them off temporarily, as it is possible that it may have clear the field before the saving. You can also copy/paste the custom CSS here, and I can try it as well, to make sure it is not an issue with the plugin.
If your theme has a custom CSS box by any chance, you could try that as well in the meantime.
2. Those fonts with the block layout are inherent with the theme, using custom CSS is the right choice in this case. If I may suggest a custom CSS to target those:
[html]label.asp_label {
font-size: 12px;
}[/html]
This should be the shortest solution.
3. You can change that here: https://i.imgur.com/QQyZuTh.png
I hope this helps!