Drop an embed component into the bottom of the page, and paste the CSS code wrapped with <style></style> tags. This will allow you to preview it in the designer.
Once you are okay with the div, move that code to
Site Settings > Custom Code > Header Code
This will apply the changes site-wide, but will only take effect on published sites and cannot be seen in the designer.
Here is a nice site, where it lists the easy to use css styles which you can recreate in Webflow by adding a class to a div, and then enter the styles shown in the example:
Some of the shapes might need custom css, but still fun