Installed theme, now site is totally janky

I installed a theme called Intelligible from Panda Themes on my site https://www.start-somewhere.com. I uploaded the theme folder directly into the WordPress install on GoDaddy using Filezilla.

When I tried to activate it, WordPress said the theme could not be activated due to a fatal error (below).

Fatal error: Cannot redeclare wp_pagenavi() (previously declared in /home/content/93/7472393/html/wp-content/plugins/wp-pagenavi/core.php:12) in /home/content/93/7472393/html/wp-content/themes/intelligible/admin/includes/wp-pagenavi.php on line 180

Not only that, when I tried to delete the theme, I got a blank page with the fatal error text. When I tried to activate a different theme, I got the same thing. Now when I just go to the public page of my website, I get the same blank page.

What happened? And how can I fix it?

Thanks in advance!!

p.s. In terms of WordPress/html/tech fluency I’m in between “beginner” and “advanced”

p.p.s. The Panda Themes support and Themeforest support (where you download the theme) isn’t at all helpful in this matter.