Strengthen .tutorial selector

In certain circumstances the tutorials.less file loads after
overlays.less - this means the tutorial overlay gets a white
background - account for this

* Add slider-button class to avoid specificity wars
* Limit styling to .button class to avoid conflicts with tutorial

Bug: 57192
Change-Id: I70f2a610d441db9d8665a3ae6c37d17d033443a6
Merge "Strengthen .tutorial selector"