Basis Theming / Sub-Theming
My tooling is:
  • Editor - Sublime Text 3
  • Browser - Chrome  53.0.2785.116 m (ish)
  • Working from Windows 10 onto an Ubuntu Server

It doesn’t really matter what the tooling is though, any code editor, modern browser, or functioning local setup will do fine.

Setup site for dev settings:

Turn off CSS/JS aggregation

Go to Configuration > Development > Performance
These should be unchecked:

Turn on Theme Debug mode

Open a file in your active config directory (by default files/config_*****/active )
Open system.core.json
Find theme_debug and give it a value of 1 

Add basic site content

Go to Site information by going to Configuration > System > Site Information


Add your logo, favicon, and slogan if you have one

Creating a theme

Make new theme folder, generally the folder is named after the theme’s machine_name, which cannot have spaces or dashes, and generally replaces spaces with underscores.

Then create a .info file, this has to use the machine_name of the theme. In my case I’ve created the a new folder in themes/ called dogs_are_awesome/ and my info file is dogs_are_awesome.info.

So the full path of my info file, from Backdrop’s root, is:
themes/dogs_are_awesome/dogs_are_awesome.info 

In my info file I’ve added the basic information needed for a theme:
type = theme
name = Dogs are Awesome!
description = The theme for my site, based on Basis
backdrop = 1.x

Now that we’ve made a theme, let’s enable it by going to Appearance > List Themes

Find your theme, and click the “Enable and Set Default” link under it’s name.