Why use CSS overrides?
It's always a good idea to keep your CSS styles separate from the existing template styling, if you're using a pre-built Joomla template.
It's easier to maintain
Your changes ...
There are several ways to add custom CSS code to a specific page. Here's a couple of alternatives:
Alt. 1 - Page class
Use page classes. In the menu item editor, under the "Page display" tab, there's ...
.deeper is applied when the menu item has a submenu (item)
.parent is applied if this menu item is parent of another menu item
If you have a menu showing all levels then both classes are applied to a ...
You don't overwrite core files. You never do. Just don't.
Put the CSS files into your template directory and load them from there.
Have a look at how the Protostar template does it. The CSS file is ...
what steps would you take to ensure that the look and feel works in synergy with Joomla and other templates?
There's no way of knowing what will, and what will not, be supported in a template so my ...
You can just add a view override in the template:
Understanding Output Overrides
Component Output Types and Layout Overrides