Menus and Other Questions

I’m trying to update my website to a new theme and I’ve been doing some customizations, but I have a couple of issues that I’m trying to resolve, but can’t seem to figure out.

While all the demos of the Frontier theme show there are multiple below-logo menus, I can’t seem to set up the menus in the Menu section to permit multiple menus displayed there. How do I do this?

I use external images (stored on my site, but with URL’s) for most posts on my site. I had to get an WordPress plugin to enable me to use URL’s for Featured Images, but when I do that, they only show the black X showing the image wouldn’t load in the Slider. How do I get this to work?

Speaking of the slider, I removed the “default image” for the slider, but instead of using a blank background for posts with no images selected, it’s still showing the default Frontier image for the slider. How do I fix this without uploading a different image?

The final question deals with CSS in the layout. I’m currently using Jetpack to add CSS (I know Frontier permits this, but I want to keep the data should I swap themes without having to copy it out every time I make a change). When I try to make adjustments to the formatting of the .post element, (such as font-size), it only changes the post header data (like # of comments, post date, etc.). How do I get it to modify the rest of the text (It seems everything’s font size 12 or larger and that’s just too big for me). I also want to know how to modify the backgrounds of widgets, either individually or as a whole. I can modify the background of the sidebar column, but cannot modify the individual widgets. Is this something I can fix with CSS or is there something more dire I need to do to accomplish it.