move css-related code to template

CSS support file import, which means one css file can include other css
files, thus we can simply code by making the "main.css" file as the
default style for all themes and templates.
add a mustache-file-render function

mustache-file-render is a shortcut function inspired by django
render_to_response function, it will render a file with a hashtable
context and then returns a string.