This is a known bug with version 8.5.4, please update your site to a later V8 version and this error should be resolved.
1/ Click on the "Cog" icon on the top left of the concreteCMS admin bar, from the menu select "Design" and then click "Customize".
2/ From the customize options scroll all the way to the bottom until you see the "Advanced" section, under that click on the "Cog" icon that is next to the words "Custom CSS" in the CSS window that appears paste/type your CSS code:
3/ Click on "save changes" then click on "Entire site" and publish your page.
If we have asked for your environment info, you can find it by logging in to your site and going to the following location:
Dashboard > System & Settings > Environment > Environment Information
Alternatively, in the fast search bar, you can type "env" and a link to your environment info will be listed.
