Nextcloud Theming bug?

Hey everyone!

I got some strange behaviour in nextcloud 11 theming.

I created a folder called “mytheme” in the /var/ww/html/themes/ directory. Then I declared the theme in config.php.
So far so good. The Style of the Login-Page still looks the same because I havent added/changed any files.

Now I created a blank “styles.css” file in /mytheme/core/css/ and suddenly the login-page changes the style and the “login” button disappears without even adding anything to my “styles.css” file.
Any idea?

Before it looks normal like nextcloud 11.
After creating a blank “style.css” in my core/css folder in mytheme, it looks like this: