reduce space between widgets twenty fifteen

and is there a way to change the size of the fonts in the sidebar?
site
https://beenhakkers.nl

and the title of the pages also, the fonts are huge and eat a lot of space

I’d like to reduce the space between the widgets in the sidebar of twenty fifteen also.

@media screen and (min-width: 59.6875em) {
.main-navigation {
margin: 0 20% 10%;
}
.social-navigation {
margin: 0 20% 10%;
}
.widget {
margin: 0 0 20%;
padding: 0 20%;
}
}