How to do "upgrade friendly" customizations?

[Steve]

Hello all,

I was wondering if somebody could point me in the right direction. I have just installed Prestashop 1.7.3 with the default "Classic" template and I want to make some modifications. I want my changes to remain after any future upgrades. I am comfortable with HTML and CSS and I've used PHP includes before too but I am not familiar with Smarty or Symphony.

1. I found the "custom.css" file and have made some small changes there.  During a version upgrade, would that file be overwritten or will my changes be safe?
2. If I make some global changes (eg - background colour of buttons) via the theme.css, would those changes remain after an upgrade? Or do changes in "custom.css" override the CSS in theme.css, in which case I would make the changes there?
3. When I look at any template file, there is a warning that if you want to upgrade in the future, you should not edit the file. It indicates we should look on prestashop.com for info on how to customize ... but I'm struggling to find help.  For example, if I change the text "Shopping Cart" in the cart.tpl file to "Shopping Cart - please select $CAD" , would that be overwritten if I upgrade?
4. Is this where "overrides" come in?

I'd really appreciate some direction!

Thank you!

Steve

EDIT-

If anybody has anything to add to this topic, I'd appreciate it.

I came across this information https://developers.prestashop.com/themes/smarty/parent-child-feature.html which got me started.  I think it's working but I really didn't understand the bottom part of the instructions (attached).  I believe that If I want to modify a template, I just have to put a copy of that template in the corresponding "child" theme folder and edit it there. However, I didn't understand: " ... but most likely you still want to extend product-list template." What does it mean to "extend"?

Edited March 29, 2018 by Steve
Nobody has responded but I've made some progress on my own that I wanted to share (but I still would like help) (see edit history)