the documentation said to use scss.
The main documentation that you should use is the Helix Framework documentation. Separate templates documentations are a little addition.

Helix also has SCSS, but this is for advanced developers who create Helix -based templates. For simple user it is not required.
More than 1 hour ago I changed the image on the home section, cleared the cache several times. Strange that the image hasn't been changed yet.
There is not any code for image in your custom.css

Or what you mean?