Newbie questions about the theme "deliver"

I am unfortunately relatively new to Grav and am currently trying to build a website using the “Deliver” theme. Currently I have three problems:
1.) On the project page you can see a showcase, which I would like to use as well. Unfortunately, it is not shown. I have already inserted a corresponding image on the page and installed the slides array, as well as set the header_options parameters in the site.yaml. What is still missing here?
2.) I would like to change the basic fonts/colors - I have added the variable “$core-accent: #000099;” to _custom.scss as a test. Unfortunately, this change is not applied either. I know that I have to recompile the scss files, but I only have a webspace without direct shell access and I also read that this happens automatically when changing the _custom.css?
3.) I have included an openstreetmap via the leaflet-plugin. Now I want to add a circle to the map - how would you best proceed?

Thank you in advance!