Prestashop 1.7 list view on custom theme

[Danny Lee]

Hi...
I created my e-commerce with Prestashop 1.7 and I bought a theme online and all works perfectly but, now I want to use the category grid view like a "list view" and not a grid.
I tried to follow MyPresta instructions to do this one, but it doesn't work (cache cleaned and force recompiled all). 
My dubt is specially on custom.css file. The procedure tell me to modify it but in the theme I bought, there isn't custom.css but I have only a theme.css . I tried to modify it like I should have to do with the custom.css but without success.
I see the two icons (grid/list), they're clickable, but nothing change.
Could anyone help me? Thanks a lot