Have discussion with @timdream  about import inlining and filtering. He suggest to explicit include extra skin via css link, instead of @import inlining. That approach is more like what web developer did to apply bootstrap theme.
And we could only do the filtering for different layout (phone, tablet) type. In build stage the build script will automatically concatenate css files into single file.

so we only need to include `skin.default.css` in `header.css` and keep the rest skin separately.