February 10, 2016 
Today I received an email about some strange behavior on the TFS 2015.1 dashboard. The partially succeeded build bars did not show in the widget. A colleague (Thanks Edwin v/d Thiel !) figured out what was wrong.
The stylesheet class that was referenced had a casing error. When looking in the code you see that the style references the partiallySucceeded class.
but in the stylesheet (widgets.css) it says partiallySucceeded (with a capital S).
To change this open the [widgets.css] located in the [Default] and [HighContrast] folder:
C:\Program Files\Microsoft Team Foundation Server 14.0\Application Tier\Web Services\_static\tfs\Dev14.M89-Part7\App_Themes
Change the class, and save the widget. After that (CTRL-F5) it shows tha partially succeeded bars again!
Update 11-02-2016 (thanks Mario!):
There are three classes that needs to be changed; .bar.partiallysucceeded, .bar.partiallysucceeded.hover, .bar.partiallysucceeded.selected
If you are using Chrome, CTRL+F5 will not do the job, a CTRL+R is what is necessary to refresh the cached css.
Update 2 11-02-2016:
This is a warning !
Sometimes TFS updates do not always handle the situation well when files (particularly .CSS files) were changed manually on disk. The result is (sometimes), that the TFS update was “successfully” applied, but the once CSS file did not get overridden (obviously because my manual changes had a newer time stamp than the files installed by the updated
The Road to ALM 
Top tip! I haven’t noticed it before, however thanks. It worked for me. There are three classes that needs to be changed; .bar.partiallysucceeded, .bar.partiallysucceeded.hover, .bar.partiallysucceeded.selected
If you are using Chrome, CTRL+F5 will not do the job, a CTRL+R is what is necessary to refresh the cached css.
I saw that on January 5th, but didn’t found out where to report that. Do we have an official tool to report such kind of bugs from TFS?
You can report this on connect.microsoft.com
It happens that I just installed TFS 2015 Update 2 RC 1 and the problem is solved. I can see that now the widget is using a lowercase CSS class name.
Cheers
XRef’d https://connect.microsoft.com/VisualStudio/feedback/details/2137793/build-widget-does-not-display-partially-succeeded-tfs-2015-update-1 for completeness.