Use separate highlight styles for different platforms

[6a68]

_This might be a little too ambitious for a good first bug, so if you are a new contributor and are interested in taking this on, please ping me to get guidance before you start. IRC is best (my nick is 6a68), or leave a comment in this bug.

Right now, we style the recommendation the same way for all platforms. In general, this is fine, but it would be nice to have platform-specific highlight behavior when the recommendation row is selected.

Here's a sketch of the tasks required:

• Create individual files in the chrome/skin/ directory for mac, windows, and linux.
• In each file, copy over the highlight style for each platform (see this comment for links to files). You want to take the .autocomplete-richlistitem[selected=true] rules, but apply them to the #universal-search-recommendation selector.
• Modify the stylesheet loading code to add a check for the platform type, then load a second, platform-specific highlight stylesheet.
• You can get the platform type by checking the value of Services.appinfo.OS. The possible values of interest are 'Darwin', 'Linux', and 'WINNT'. (It takes about 4 hops in MDN to track this info down, directly inserting the answer here to keep the bug manageable overall.)
• Once the changes are made, restart Firefox and see if the recommendation highlight matches the highlight of the other rows in the autocomplete dropdown. If it looks good, open a pull request. Don't worry about checking all three platforms; I can check that.

[kaganjd]

starting to dig into this one!