I am trying to find a "Fluid Theme" for a client.

Your question isn’t very clear. However there is quite a lot of “fluid” or responsive themes around now.

Check these “responsive” themes in the WordPress theme directory: https://www.ads-software.com/extend/themes/search.php?q=responsive

Or try “flexible-width” themes here: https://www.ads-software.com/extend/themes/tag-filter/

Otherwise just do a search for “responsive wordpress themes”