I was a bit rusty after not using the system for about 21 months after the old version closed down in march 2019, but i was begining to pick things up quite easily with my new site and wanted to experiment with the css.  Only I seem to have made a mess of things.  I downloaded  a css theme and was  just starting to look at the different layout options, but now navigation menus now keep collapsing - as wysiwyg they appear as just a tiny narrow bars and  the menus appear briefly when i put my mouse touches a bar but then the list toggles off again before i can select an option.  I've had a look at the background code but can't decide what should be there or can be deleted.  Some of it looks superfluous but i'm not confident that deleting some of the code will solve the problem

I need to get back to the cms