If I would like to create the custom theme… What are the general suggestions or limitation?
Is it a good idea to start with Bootstrap based theme or it is a bit overkill?
This is what I am studying now. Publii themes are a bit more complex than your general wordpress or Jekyll theme, as they also contain all the “logic” for the theme settings that one would allow in the Publii application. I would suppose you could make a theme that is fairly static and have very little modification capability through the Publii application interface. In that case – start with the “Blank” theme found in the Developer Documentation.
I do not think it is impossible to use existing frameworks (like Bootstrap or Bulma) for theme development, but they are not use in the available themes for Publii as near as I can tell.