Bring in jQuery theme styles with CSS @import rule

Precompile the theme CSS file separately.

While this approach results in a separate HTTP request, it has the
advantage that the relative paths specified for images should just
work (and not require conversion to use ERB/SCSS to make use of
Rails asset helpers).

Task # 5422.

Signed-off-by: Alex Coles <alex@alexbcoles.com>