While all the demos of the Frontier theme show there are multiple below-logo menus
What do you mean by multiple menus? This theme only has 2 menu locations one on the top bar and the other below the header. Also, which demo? Besides the preview on wordpress.org, I don’t know of any demos.
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
The slider included in the theme is really just a very simple slider designed to work with the normal featured images. Having said that, if the plugin you used (since you didn’t mention which) does indeed set an image url for the featured image then I don’t see why it shouldn’t work.
You may be better off using a dedicated slider plugin.
When I try to make adjustments to the formatting of the .post element, (such as font-size), it only changes the post header data
You may be using the wrong IDs and Classes.
For the content on the posts page use
For the content on single posts use
I also want to know how to modify the backgrounds of widgets, either individually or as a whole
For the sidebar widgets you could use this:
Or you could target a widget individually by using its ID. Ex.
You may find this useful for these types of customizations:
Customize WordPress Theme CSS using Firebug