The discovery pane opening links in Firefox as you’ve described is a bug, it’s already been fixed and the fix should go live in TB62 beta and TB60 ESR when those come out.
In the meantime:
- Click on “Themes” on the left-side menu in the Add-on Manager.
- You’ll see “Looking to personalize your browser? Choose from thousands of themes.” at the top. Click on the second sentence.
- That will open the Theme page inside Thunderbird. Each theme listed here has a little “Add” button under it on mouseover, which will also be the case if you Search in the top right.