which is most simple yola style and best for using custom css?

If you mean which is the best Yola style, to add custom CSS to, then I'd advise starting with the "NoFrills" style.

There are a few options if you would like a "blank" style. The following styles are plain white with background color options:

https://www.yola.com/tutorials/articl...

CleanSlate
BareNecessities
NoFrills
SqueakyClean
RedPlanet (To make the style blank, create a plain white banner and upload it. You will then have what looks like a plain white page).
These styles can be found in the Free Styles section of the Style Library (click on the "Style" and select "Change Style" to see the style selection).