Well, yes and no.
In the context of the Theme there are some fiels that are loaded "by convention".
Or for a module if the file is named module.css
For all other css files you should use the Client Resource manager to load them: https://docs.dnncommunity.org/content/tutorials/client-resources/index.html

Perfect! I was already using Client Resource manager but I like module.css better

I believe you are looking for #5. Rename the file to module.css and move it up one directory. The module.css is not isolated to just one Web User Control, it will load on all user controls within the module. I've had trouble in the past doing this after a module is already installed and sometimes takes removing the module from the Extensions and re-adding it to see the module.css file.

I don't believe there is isolation to just one control unless you specifically load the .css file by name.

<%@ Register TagPrefix="dnn" Namespace="DotNetNuke.Web.Client.ClientResourceManagement" Assembly="DotNetNuke.Web.Client" %>
<dnn:DnnCssInclude id="cssHomePublic" runat="server" filepath="~/DesktopModules/CVContent/HomePublic/HomePublic.css" />

Happy DNNing

yes, it was #5 but I didn't catch that when reading, I thought I still had to specify the full path and so on, I'm looking for automation so, module.css it is... thank you

Ryan's reply will work too, but it's more work to do and maintain.  You should always use the module.css, unless you have a good reason not too.  It just works and all you have to do is be sure the file is in the correct place with the expected name.  Done, and done!  😎 

Yes, that's what I had working but I like module.css better... thank you