Clean up the CSS for File module

Part of the CSS Cleanup: http://drupal.org/node/1089868

Overview of Goals

1. Make it easy to remove unwanted design assumptions in the theme layer, while maintaining critical functionality (such as functional JavaScript widgets).
2. Prevent uneeded administrative styles from loading on the front end.
3. Give modules the ability to include a generic design implementation with their module, without burdening themers.
4. Make CSS and related markup more efficient and less intrusive to improve the themer experience.

The CSS Clean-up Process

Use the following guidelines when writing patches for the core issues listed below.

1. Put CSS is in the appropriate file: CSS should be moved to separate files, using the following
   name spacing conventions based on their purpose:

   module.base.css

   Should hold structural and behavior related styling. CSS should be coded against the Stark theme. The absolute bare minimum CSS required for the module to function should go here. If there is no CSS required, this file should be omitted.

   module.theme.css

   Should hold generic design-related styles that could be used with Stark and other themes. It's where all design assumptions like backgrounds, borders, colors, fonts, margins, padding, etc, would go.

   module.admin.css

   Should hold styles that are only applicable to administrative pages.

   To see an example of this in practice, look at Drupal's system module.

2. Remove Assumptions: Styles that make too many assumptions, introduce superflous margins, padding and add things like font settings are not necessary and don't belong in core module CSS files. In cases where core themes depend on these properties, they should be moved to the CSS stylesheet of the respective theme.
3. Reduce Selector Specificity: CSS code that resides in modules should be written in a way that's easily overridable in the theme layer. To improve the Themer Experience and make core CSS more efficient, CSS selectors should be made as general and short as possible. For example:
   • Use .style {} over div.style {} where possible.
   • Use .module .style {} over div.module div.somenestedelement .style where possible.
4. Don't use IDs in selectors: Use of ID's in core CSS selectors requires more specificity in the theme layer, making it harder and more annoying to deal with. It makes achieveing consistency in complex design implementations much harder than it needs to be. We need to stop making life hard for theme developers.
5. Don't be afraid to change markup: There's lots of overlap between using proper and semantic markup and doing CSS right. If you come across a case where CSS is being applied where using a more semantic elements would solve the problem, then change the markup in your patch to make it right. For more information, see the Drupal 8 Markup Gate rules.
6. Start with Stark and cross-browser test.
   1. "Design" markup and CSS for the Stark theme.
   2. If applicable, adapt the styles to match the core themes afterward.
   3. Finally, test the changes in all supported browsers and ensure no regressions are introduced.