Customize our site styling

See here on the documentation: https://support.getgrist.com/self-managed/#how-do-i-customize-styling

Come up with a color theme for the site starting from this template and use:

-e APP_STATIC_INCLUDE_CUSTOM_CSS=true \
-v /LOCATION/OF/THE/custom.css:/grist/static/custom.css

Come up with an xplain icon and include in custom.css with eg --icon-GristLogo: url("ui-icons/Logo/GristLogo.svg")