Edit Default Theme Help

I am trying to edit the default theme. I uploaded 2 screenshots so you can see what I am trying to do.
Screenshot 1: https://pcpagedesigns.com/Capture-1.jpg
https://pcpagedesigns.com/Capture-1.jpg
The first graphic shows the current theme layout.

I would like to make the layout to look like the second graphic screenshot. I think its a table. See screenshots and you will see what I am trying to do. I want all of my thumbs to fit across 100% of the page not 50% of the page. Can some one help me locate where I would edit the layout.
Screenshot 2: https://pcpagedesigns.com/Capture-2.jpg
https://pcpagedesigns.com/Capture-2.jpg
Thank you!

Dingomann wrote:
Hello ... U're going to change the default theme but ...what's your nopCom ver.?
Best.

I am using Version 3.9 - Its the most stable of them all so far. Any help is appreciated!

Dingomann wrote:
... you can try that https://www.w3schools.com/bootstrap4/bootstrap_grid_system.asp
Best.