Does NGE uses current css styles?

Each part (widget) uses the current css by default (css from the template).
For example the "Tab" part uses the default bootstrap css.
You can define your own styles. To do so, use the "custom style". The colors you choose in the screen options (border, background...) will generate specific css for this part.
You can also use a class name. It will be added to the part main tag <div class="myclassname"> and so the css defined in this classname will be applied to the part.

So : generally, in each part options :
Style =
default => uses current template css
custom => you define your own colors and own css
class => will use a classname