onigiri grid customisation

nice theme, great work.
I know you don t support customisation on your template but i just want a
hint or some links to read for editing the grid based on the jetpack portfolio.

+ i want to edit the whitespaces between the thumb nails
+ maybe implementing vimeo videos instead of a thumbnail

where i can find the core css or js file to edit the space between
the images? how does the whole stuff work?
maybe you have a tutorial link or a small small hint
to start my research. googelt 3 hours yesterday with no result

UPDATE:
+ got it on line 2338:
.jetpack-portfolio-shortcode .jetpack-portfolio {
display: block;
width: auto;
margin: 6px;

+ now i want to try to center the whole grid?
how can i do that?

thanks so much
kindest regards

matthias