Different pages/posts are using different themes

I was trying to add a login/register link on my blog. I am quite new to this and wasn’t sure how, so I installed a different theme to see if it was the theme or one of my settings. The new theme had it, but I didn’t like it, I like the old theme.

I deleted the them that I didn’t like and found a plugin that did what I want it to. Now some of my pages are using one theme and some are using the other…I have checked in my control panel (of my server), and the theme appears to be deleted. Why then is is showing up half of the time.

It appears that it is my “pages” that are using the theme I don’t like and my posts are using the one installed.

Thanks in advance for your help.
J