How to customize the theme?
If you go to Design -> Customize, you'll be able to change the style for most of the elements (header, footer, headings, body text, backgrounds...)
How to apply the Masonry Layout to your portfolio?
How to change the number of columns of your portfolio?
First, make sure you have applied the "Fresh Thumbnail Grid" custom template to your Portfolio (Page List block).
Next, click on the block, select "Design & Custom Template" and choose one of the available custom classes:
"items-per-row-1" -> 1 column
"items-per-row-2" -> 2 columns
"items-per-row-3" -> 3 columns
"items-per-row-4" -> 4 columns
"items-per-row-6" -> 6 columns
How to force all thumbnails to be the same size?
If you need to use the "Fresh Thumbnail Grid" custom template, but want all thumbnails to be the same size (ignoring the thumbnail size specified in the Page Attributes), simply select the "fixed-size" custom class in your Page List block.