There isn’t a tutorial on this exactly, but for now the very concise instructions:
• Create some color tokens, like Background color (white), Text color (black), etc. Use these systematically throughout your design.
• Create a global variant group Mode and a global variant in it Dark mode
• Edit the color tokens to look different in Dark mode, such as Background color now being black and Text color being white.
This should be all you need for majority of dark mode. For the remainder, you can directly edit components according to the global variant.