Add better color for different themes for covered/not covered lines

[ShadowIce]

I'v improved the colors because with dark theme it was not really usable.

I also had to change the version for Microsoft.VisualStudio.Threading as I don't have the originally referenced version.

[OpenCppCoverage]

Hello,

Thank a lot for this pull request. Indeed, the colors were not usable with the black theme.
I took your code and did two changes here:

• I used xml to generate pkgdef file as describe here
• Unregistered ThemeChanged event

Thanks again for your pull request, it was helpful.

OpenCppCoverage

[ShadowIce]

For some reason I totally missed the VSIX color compiler...

I'll close my PR. Thanks for adopting my changes.