Me thinks this here is a simple matter of themes. i.e;
Your var HAS been highlighted, but it just so happens that the 'highlight-color' for that particular string type is white, as styled in your current theme. Just install another one and you'll be set.
Ways/options to get them[in case you didn't know how]:
Search & Install (direct) via File > Extension Manager > Themes.
Search google for 'brackets themes' OR 'dark brackets theme' etc,
find, download and then install it manually.
Browse/Peruse through this site here
https://brackets-themes.github.io/. See one that's to your liking?
Download and then install manually.
PS: I prefer and use option 2 coz A, you have an almost endless list, ranging from official releases to random personal user created ones, from which you could choose., and B, you have a visual idea of how the theme looks before downloading, courtesy of Google Images and you are thus more likely to get one that tickles your fancy.
There maybe some other ways not mentioned (idk), but I'd suggest you start with any of these first. If still not satisfied, you could just create your own. Broken down, the process is pretty straight forward as it's just basically(if I may) HTML & CSS.
Cheers mate, happy coding!