The results of Site Renderer does not validate (CSS)

When I try to validate the css of a document produced by doxia site renderer I get a validation error: "File not found ...". Here is the page that I try to validate:
http://people.apache.org/~dennisl/commons-lang5/userguide.html

Here are the validation results, having set the css level to 3, which is as high as it can be set:
http://jigsaw.w3.org/css-validator/validator?profile=css3&warning=2&uri=http%3A%2F%2Fpeople.apache.org%2F~dennisl%2Fcommons-lang5%2Fuserguide.html

The default-site.vm has this on line 286:
@import url("$relativePath/css/site.css");

However the file css/site.css is never created by either site renderer or the 3 skins that are available.

I see two possible solutions for this. Either remove the line mentioned above, but that might have backward-compatibility issues, or add an empty "css/site.css" file to the site build.