Unfortunately, we do not have a lot of documentation on this yet, but here is a starting point.
The HTML pages are written using a templating language called Liquid. It was created by Shopify, and so for now, the best place to find documentation for this is at Shopify's site. Beware that we would not have exactly the same tags, filters, and objects. But it's close. The best way to find out what is available is to look through our official source.
Our CSS is written using a language called SCSS. Any regular CSS is valid SCSS, so if you do not want to learn SCSS, don't worry. You can just write CSS, and it will work. Files with a .scss extension will automatically be preprocessed using the SCSS compiler, which will translate it into CSS that browsers understand.
As long as you have not made any changes to your theme, you will automatically get updates to your theme as we make them. We fix bugs, make small improvements to layout and rendering, or add entirely new features that require changes to both the site theme code and the backend.
Once you make changes to a file, you will no longer automatically receive updates to that particular file. You will still receive updates to all your unedited theme files.
This can cause problems when we do make changes, and thus you need to be careful when you make changes, and aware that you will need to factor in the cost of staying up to date with our ongoing changes.
We have not yet developed a process for handling these changes, so, for now, you will have to bear with us, and we will do our best to work with you.
The most common thing that you will probably want to do is make changes to the site's stylesheet. For this reason, we strongly encourage you to NOT make changes directly to the theme.scss.liquid theme file, but instead to add your changes to the custom.scss.liquid file. In here, you should be able to override anything in theme.scss.liquid, while still continuing to receive our changes.
If you do not do this, when we do make changes to the HTML that requires changes to the CSS, you might end up with a very weird looking site, because the HTML changed, but not the CSS.
You've been warned!