I built the Snipcart add-on in an way that works directly with a bootstrap based theme, focusing on the default Elemental theme, but still with HTML output that is pretty generic so it doesn't look out of place in other themes.

A product block for example is really just a div, with an h2 for the product name and normal paragraphs and links for the rest of the content. So it'll really just look like what those things look like in that theme as if they were output in a normal content block.

However what you _want_ it to look and what it ends up looking in the theme could be two different things. Unless the Equinox developer specifically included styles to treat the product and cart blocks a certain way (which is unlikely), they'll just look fairly simple.

You could after installing both further customise the theme with some custom css to target these blocks, there should be classes on all the elements there ready to go (I did build the add-on with customisations in mind), it just comes down to your goals for stylings. You can do the customisation either via the theme, or using the CSS customisation feature that the Snipcart add-on includes. If you did deeper int the Snipcart configurations, there are further options to specify things like classes on button blocks, without having to provide custom block templates. But ultimately you can override the block templates too, customising them however you like. (Pretty easy for someone who knows a bit of HTML)

The actual pop-up cart on the other hand is completely independant and will simply look as per the default Snipcart styling.

What the marketplace/PRB tests are for are to check that packages install and uninstall correctly, they've been built correctly for concrete5 and they work as described. If themes and add-ons are built correctly, they should _generally_ work well together, but there's no 100% guarantee that they'll work together _exactly_ as you'd want - every combination of add-on can't be tested and people have different expectations anyway.