Why does the theme keep going back to Sultantheme's every time I change pages?

I normally use the classic theme. I noticed it looked weird today, and when I checked it had changed to the other one (“Sultantheme’s Responsive vB3-blue”). I changed it back, but as soon as I moved to any other page, it switched back. Am I missing something?

Somehow the Dope seems to be trying to change everything to Sultantheme. Go to your user CP (look at the top left of the page.) Scroll down to “Settings & Options.” Click on that and scroll all the way down the page to “Miscellaneous Options.” Look at “Forum Skin” to make sure it’s set to Straight Dope v3.7.3 and that will fix your default setting.

Click on this link, then try to go to different pages here by clicking on links (do not load a bookmark):
https://boards.straightdope.com/sdmb/forumdisplay.php?f=2

If that works, then your bookmark or whatever you are using to get here is missing the https at the front of the link. Update your bookmark and you should be good to go.

Thanks, that worked!