Improve Syntax Highlighting Themes and Add Theme Switch

[Process-ing]

Description

This PR improves the current syntax highlighting color schemes (specially the dark theme), and also adds a theme switch, improving user control over the tool's appearance. Further suggestions (especially for the color schemes chosen) are highly appreciated.

Screenshots

Changes Made

• Altered the color schemes for the syntax highlighting on both the dark and light theme.
• Added a theme switch, which allows to force the interface to use the dark or the light theme, or even use the browser's default. The option chosen is stored inside the browser's local storage, to persist between sessions.
• Updated some references in the README