Here at the Irreal bunker, we’re stodgy, like things the way they are, and are not inclined to change things willy-nilly. As far as Emacs is concerned, that manifests itself in the eschewing of themes. The bunker doesn’t really have a theme. It’s just the default light configuration with the background changed to a light tan (oldlace) and the cursor colored red. That’s just a couple of init.el statements; not a proper theme.
Still, lots of people like to experiment with different themes and change them often. That’s understandable—at least if you’re not stodgy—because there are a lot of great themes available and more are being released all the time.
That brings up the problem of changing themes. It turns out that when you load a new theme, it overlays the existing theme and unpleasant interactions can occur. The proper way to handle this is to first disable the old theme and then load the new theme.
Bozhidar Batsov likes playing with new themes but finds that it’s a chore to worry about disabling existing themes. So, being Batsov, he solved that problem by writing a bit of Elisp for loading a new theme that first disabled any existing themes. It’s short and easy and if you also like to experiment with different themes, it might be worth your while to take a look at Batsov’s solution.