Comment on Stallion Responsive WordPress SEO Theme by SEO Dave.
Stallion Responsive includes 26 widget areas (plus an unlimited number you can make via a Stallion feature), there’s 5 widget areas related to the header area, several of them could be used to achieve what you want by adding a widget with an optin form to a particular widget area.
These are the widget areas around the header area.
Above Header Area : Above everything.
Within/Over Header Area: This one floats a widget over the header area to the right when the search form is disabled (theme feature to disable search form), basically replaces the search form (though wider than the search for: designed primarily for adding AdSense ads over the header).
Below Header Area 1 : 970px wide below header and main navigation menu.
Below Header Area 2 : As above except it comes after the photo navigation menu and the features slider (two theme features).
If looking for full width of webpage the two above are what you want. If you want the optin form to be the width of main content, the next option is better.
Below Banner Image Area : This is the same width as the main content and directly above the main content. Banner images is an optional theme feature, so if you use the feature it’s below them followed by the main content, if you don’t use banner images it’s directly above main content with the header above that.
As you can see there’s plenty of ways to add content. Stallion has a widget display feature so you can set where a widget should load, so if you only want the widget to load on the home page you can do that, want it to loaded on home and a few select pages/posts can do that as well or want it only for posts in a particular category, can do that as well.
There’s also a shortcode system for adding widgets anywhere within content, so if you had a static home page (WordPress core feature to use a Static Page as the home page) you could put the widget anywhere in the main content. Got an example at https://stallion-theme.co.uk/stallion-responsive-theme-recent-comments/ scroll down to just below the Google search box and the Latest Comments output is a widget added via the widgets anywhere feature , lower down the page is another widget for Most Commented Posts.
Regarding the Thrive Leads plugin I’m not familiar with it, though I don’t see why it wouldn’t work.
You can download (zip file links at https://stallion-theme.co.uk/stallion-responsive-theme/) and install the full version of Stallion Responsive and run it in Demo mode for testing (in Demo mode main options reset roughly every 500 pageviews). I would try it out and check everything works as you want it, if not you only wasted your time.