Customize theme display post embed

Halo!

I’m new to WP (but not new to coding/php). I’m still wrapping my head around how the whole system works and working with existing plugin/themes. The problem is sometimes I don’t even know what to google(like now)..

I’m using UserSubmittedPost(USP) and have achieved few things:
-Filter their submit form to add some fields
-Add meta to the submitted post, from that new field

The new field is a dropdown (Text,Image,Audio,Soundcloud,Youtube), and the custom field containing the embed code if SC or YT selected.

So these part is manageable. I can understand plugins(functionality) more than Theme(general flow of data and displaying them), it seems.

The last part is displaying the post based on the selected embed type in GRID. List these posts in grid, each grid embeds the posts.

Currently I just copied their shortcode and modified it, so the html generated will also include the necessary embeds.

Then I wanna grid this. I wanna just get a Theme that has grid and work from there, so it’ll use the Theme’s styling. But most tutorials show grid/gallery with image thumbnails, some involves add_theme_support(but there’s no soundcloud for this param, right?).

I slapped the shortcode on the page’s text, but the html it gives has nothing to do with the current theme this way.
Grid-ing involves some css, so I should find the grid layout in the Theme used and modify that? And… (this is the main question..) where would I find this in the theme? Or, what resource can I look into for something like this?

I have no problem learning how to write codes for this and that, but working with existing theme requires understanding of how the theme is set up(and I assume not all theme creators put things the same places?).

Cheers!