Issue with the theme

Hello!
I am very grateful to you for your training videos on DRUPAL, but I have encountered a problem. I accidentally deleted a theme, and I can't restore it. I'm downloading it again, inserting it, going to my site, it loads slowly, and in the end, it gives an error either in common.inc or theme.inc. If I delete it, the site loads quickly and smoothly.
Thank you in advance!