I suggest that a link and icon for "Themes" be added to the "Customize Toolbar..." dialogue be added to the Firefox Toolbar, enabling the Themes menu item that appears in the Add-ons Manager. The following describes the current process required to achieve the needed result:
I am chasing a bug in Firefox regarding the theme that is displayed when Firefox restores one (or several) of my bookmarks, which I load manually from the Bookmarks Menu. At startup, the correct theme (i.e., last manually chosen) is displayed; when the next tab is added from the Bookmarks Menu, the theme is changed to a Dark Theme, even though the previous theme is listed as the current theme. As a result, I need to go to the Extensions icon on my Firefox Toolbar, load the Add-ons Manager from there, then choose Themes from the menu on the left side of the Add-ons Manager, and adjust the theme from the one that has been erroneously used by the Bookmarks Menu item that I had selected.
The repetitive process described happens daily. A customizable icon on the Firefox Toolbar for "Themes" would greatly simplify this daily process, until the bug can be found and remedied.