This fixes the following deprecation warning
```
DEPRECATION WARNING: Extra .css in SCSS file is unnecessary. Rename /usr/src/app/app/assets/stylesheets/darkswarm/variables.css.scss to /usr/src/app/app/assets/stylesheets/darkswar
m/variables.scss. (called from _app_views_layouts_darkswarm_html_haml___3990351141569700945_46957256630860 at /usr/src/app/app/views/layouts/darkswarm.html.haml:18)
(...)
```
"admin/all.css" which imports these SCSS partials already imports the
color variables in "admin/variables", so actually there should be no
need to import the variables again.
However, "application.css" calls "require_tree", which means asset
precompilation through Sprockets would attempt to compile each of the
SCSS partials individually. When compiled individually, the color
variables are not available to these partials.
This is a quick solution to allow precompilation of "application.css" to
complete.
