Refactor - Configure colours through prefix config variables

[itslukeDev]

Description

I really liked this snippet and wanted to experiment with different colors to find something that would match my theme a little better. I found that it was kind of tedious to find and replace 11 references to a color variable for each prefix group.

This PR is the result of me reworking the CSS to only define the prefix's color in one (technically two) variables to make color swapping a breeze.

Changes made

• Created prefix variables under the light and dark themes that are used instead of the color variables for each prefix group.
• Added a VSCode snippet to quickly add new prefix groups.
• Heavily refactored the README to shorten paragraphs into step-by-step bullets and to reflect the changes made above. (I completely understand if this last change is pushed back.)

[itslukeDev]

Note to self: Actually look at the issues and pr's on a repo before accidentally re inventing the wheel
#4
#18
#14

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