Pull Request filter text too dark on dark theme


__Version__
GitHub Extension for Visual Studio version: [2.5.4.3295](https://ci.appveyor.com/project/github-windows/visualstudio/build/2.5.4.3295/artifacts)

__What happened__
Using the dark theme, select a user from the assignable users filter. Open up the filter again and notice the unreadable `Clear filter` text and `X`
<img width="138" alt="clear filter text too dark" src="https://user-images.githubusercontent.com/10103121/42833964-7e382e5c-89aa-11e8-8694-0d80383173d8.png">


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Pull Request filter text too dark on dark theme #1785

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Pull Request filter text too dark on dark theme #1785

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