Hello,
I would like to suggest you to create css folder inside _diffs folder and then put your own _custom css inside that folder. If you have main.css in docroot/css folder then copy that file in this folder and after that import your _custom css in main.css file. (docroot/css/main.css file is created by itself when you create any new theme ).
Liferay overrides its own main.css file with your main.css file which is inside _diffs folder. Styles defined inside _diffs/css folder override any styles in the parent theme. Your custom file effects only after putting it inside _diffs folder.
As an example,
I tried in my case as following.
I created 4 folders inside _diffs folder as per following.
_diffs
>css
>images
>js
>templates
then I put main.css file into css folder(inside _diffs) and then I wrote responsive code inside custom.css file and imported that file in main.css.
Shreeya Vachhani
Software Engineer @ AIMDek Technologies Pvt Ltd.
