Clean colour definitions

[C0Florent]

Hi, I've been using your theme for a while now, I really like it!

I'm trying to extract the palette out of the theme definition : instead of having a list of contexts/scopes each with a colour, get a list of colours where each has a list of contexts/scopes in which they appear.

This would make it easier for me to port the theme to other applications.

The problem is that the colour definitions are inconsistent: some are upper case hexadecimal, some are lower case, and I also noticed that editorInlayHint.background is set to #0000 which does not even have 6 hex digits.

The fixes should be trivial, but I don't know whether we should switch to all hex uppercase of hex lowercase. And also I don't what to do about #0000, I guess writing #000000 should be equivalent, but I'm not sure.