404 Page don't work after change to https

Hi, sorry, my English is poor, but i try to explain my problem. Some Time ago i change to https. All works fine. today i recognized, that the 404 page don't work. The Error is mixed content.Mixed Content: The page at 'https://www.womoknipser.de/reelaender-uebersicht' was loaded over HTTPS, but requested an insecure stylesheet 'http://www.womoknipser.de/wp-content/themes/catch-box-pro/style.css'. This request has been blocked; the content must be served over HTTPS. What to do? Greetings from Germany Madie