1 - make sure you are editing the proper css file.
If you are modifying the default template, edit the stylesheet found in the root/storefront/view/default/stylesheet/ folder.
If you created a template clone extension to modify, edit the stylesheet found in the root/extensions/yourclone/storefront/view/yourclone/stylesheet/ folder
2 - After you make a change to the css file, save it. Then clear cache in your browser, and refresh the page. In Chrome, if you are using the Developer Tools CSS Inspector extension, it will never use cache on a tab running Developer Tools, so you save the clear-cache step while working.
Usually, clearing browser cache is a reliable way to see saved CSS edits.
If this does not work, then best practice will be to clear server-side AbanteCart Cache under the System>Cache settings of the Admin Control panel.
lastly, if you make changes to a tpl file, you will find that you often cannot see the results of the change until you clear AbanteCart cache.
IOW - clearing browser cache is good enough for most CSS editing, and this step is automated by Chrome when Developer tools are on. Clearing System cache is susually required to see the results of tpl file edits.
HTH.