Warning! I tried PigsFly's idea and it worked. I selected what looked like the old theme but, when I reopened the site, the Help/Search/Profile/ etc. were gone, only the Home and Gallery links were visible. I couldn't find a way to go back to the default theme.
After a while I thought of moving the cursor along the band from Gallery towards Home and noticed the lower right corner of the page showed Profile - then I clicked and got to my profile. Finally, I was able to get back to the default theme.
It appears some of the themes have problems so this might help someone else who has problems changing the theme.
Jim