Resources -> FileSystem - Dark text in treeview

Before:
![before](https://f.cloud.github.com/assets/2838324/1797335/13c58c00-6acd-11e3-8f40-58ed9e2ba7be.jpg)

After:
![after](https://f.cloud.github.com/assets/2838324/1797334/1395f6f2-6acd-11e3-91ec-8423835147b2.jpg)


Removed duplicated code - see: 
https://github.com/mauricecruz/chrome-devtools-zerodarkmatrix-theme/blob/master/canary-theme-extension/styles.css#L2063-L2097