Interesting question, John...
Survey123 forms are not HTML and cannot be styles using CSS.
However, there are already options available in Survey123 to style your surveys. Use the Style tab under Settings in Survey123 Connect:
More hot tips for styling up your forms can be found here: http://doc.arcgis.com/en/survey123/create-surveys/styleyourform.htm
But, if you want more control over the look and feel of the different question types in Survey123, I would absolutely suggest downloading AppStudio for ArcGIS and cracking open the Survey123 source code (available as a developer sample).
This way, you can make changes as you wish and build your own custom version of Survey123.
Hope this helps,
Hannah