1.9.4

Note: --background-color-dp-XX is depreciated; use --color-surface-X instead.
The current background color CSS variables are too complex so I reduce the variables to 4 surface colors instead. It also lays the foundation of a more versatile theming system and hierarchy that will be introduced in the later version. The current --background-color-dp-XX will be kept for a few versions until all skinStyles have been reworked to use the new variables.

For other changes, please see the commit history. I am working on getting automatic changelog working but all these stuff are new to me so it might take a while 😟 I will also be tagging releases from now on as requested (#389). 🎉