Since email@example.com and more especially this commit https://github.com/apache/cordova-android/commit/73fd9e4dfa5dffde2eb14ebd6c736ca462d1e29f, the theme has been updated.
The inappbrowser plugin use images as background property of Button views.
So, depending on default margin values in the Theme, background images can be stretched.
A fix could be to use ImageButton views instead and chose a scaleType if needed.
Here is what we have today using a version of cordova-android >= 5.1.0 :