How do I get this site mobile repsonsive?

i would like to have the color changing backgrounds, but as soon as i drag my browser window to a ratio where the height is much smaller than the width, it looks terrible.

what can i do?

(Sometimes the color change does not work in the mode. Then just turn the preview on and off.)

thank you very much!

Here is my site Read-Only: LINK
(how to share your site Read-Only link)