Restores color highlighting to selected Resource in tree

Minor adjustment since #11406 to SASS `lighten()` opacity level to prevent returning pure white `#fff`. This PR brings back the highlighting of selected Resources in the tree.

Here's an example of the tree when editing the Home page:

**BEFORE:**
![tree_no_highlight](https://cloud.githubusercontent.com/assets/352182/3094202/bbd48d12-e5b9-11e3-9961-705b0878ec9d.jpg)

**AFTER:**
![tree_highlighted](https://cloud.githubusercontent.com/assets/352182/3094217/d918da36-e5b9-11e3-84a2-74549d23fe6e.jpg)