When working with shopify you will want to edit the css file for yours store theme. Editing the css file is not recommended as you will come into issues with theme updates which either remove your updates or change so much that you had not idea what your original changes were.
The best method is to add a new css file where you can override or create new declarations. I recommend you document all your changes using css comments so that when the theme does update you know what your own css overrides were being used for.
To do this you need to first edit the theme code. If you are trialling a theme then you will be restricted from editing the code until you purchase it. Once in the theme editor goto Assets and click add a new asset. Choose a blank asset and choose extension type CSS. I usually enter a filename of the store/business. So as an example pinakin.css
After this you need to add a reference to the new css file in the Layout/theme.liquid file. You will need to use the built in find facility to find the reference to .css files then after the last one add your own reference like so
{{ ‘theme.scss.css’ | asset_url | stylesheet_tag }}
{{ ‘pinakin.css’ | asset_url | stylesheet_tag }}
or
{{ ‘pinakin.css’ | asset_url | stylesheet_tag: preload: true }} – if you want to preload