Ola, the idea to never specify src in the formtemplate tag is almost correct, but I don't think it can be a hard rule.
For example, in survey, I want to template the survey question display by offering multiple templates, and allow the administrator to choose a template which will then be specified as the src in the formtemplate tag.
That said, I agree its a good general guideline, and standard-lars should definitely be made the standard form template and enhanced with CSS so the colors can be specified from a sitewide, or package specific CSS file.