Oddball problem w/ themes page

This is not a debilitating problem for the functionality of WP, but it’s a weird one that I’d like to know if anyone else has had: on the themes selection page (mydomain/wordpress/wp-admin/themes.php) the theme that is being used displays its complete code on the page. The thumbnail is still there in the middle of the code and the other themes are below that. I cannot look at any theme files because I get a ‘url too long’ error. But the theme works fine.

?? Is this not bizarre? Here’s a screenshot of the first quarter or so of the code: https://www.misangela.com/theme_page.jpg

If anyone has a clue about why it would do this, could you enlighten me?

Thanks!
Angela