In the Font Manager you only define these font-families, Helvetica, Trebuchet MS, and Amaranth you can see that on the Font Manager.
What this means is it's not the Theme nor Cornerstone is loading the Lobster font. Its the Revolution Slider, and because no preview of Slider on Cornerstone you don't get the Lobster font in there, that is why.
If you want to use the Lobster font on your headlines without the help of the slider, you can define it on the Font Manager, and set that on Theme Options > Typography > Heading Fonts
Another thing with regards to this;
That is not how you use the Element CSS feature. If you want to use the ID for applying custom CSS then you can put your CSS on the Page > CSS or Theme Options > CSS.
In the Element CSS, you don't need the
CLASS, you need the
$el, so your CSS would look like this:
Hope that shed some lights,