First you adjust the categories per row, in the admin. Go to "Configuration", "maximum values", then for "Categories To List Per Row"
However, I prefer having some padding between the categories, so I adjust the 33% like this:
On line 25 and 27 of the same file, I do this:
$col_width = floor(100/$num_categories) - 2;
$col_width = floor(100/MAX_DISPLAY_CATEGORIES_PER_ROW) - 2;
That would cause the width to be 31%, of course you could do whatever number you'd like.
There are other places where you may need to do the same thing, if you want the products to also be spaced like this, like:
modules/product_listing.php
modules/new_products.php
modules/also_purchased_products.php
modules/featured_products.php