Hello
You may add following code into the custom.css:
.navbar{ padding: 10px 0;}
.page-heading{margin-bottom:0px}
.product-images, .product-information{margin-bottom:0px!important}
.tabs.accordion .tab-title:first-child{margin-top:0}
.page-content .product{margin-bottom:0px!important;}
Here is a video tutorial how to create custom.css: https://www.youtube.com/watch?v=Qok2zRedRMY&feature=youtu.be.
With best regards
Brian Johnson