Improve theme documentation for Chamilo 1.11.x

[konaspandana017]

Added clear instructions for creating and customizing themes in Chamilo 1.11.x inside THEME_GUIDE.md.

[konaspandana017]

Hello! 😊
This pull request only updates documentation (THEME_GUIDE.md) for Chamilo 2.0.
No PHP code or functionality has been modified.
The workflow failures are unrelated to this change.
Kindly review and merge when possible. Thank you!

[ywarnier]

This proposed guide is imprecise or incomplete and suggests the author has never tried uploading a theme to Chamilo 1.11.x.

• you don't need to place the theme in both app/ and web/. The web/ location is updated automatically (based on files in app/) when using the "Clear cache" feature inside Chamilo.
• files do not have to particularly be lowercase, but they should at least replace existing files in default CSS folders (the examples cited are correct but the rule of lowercase is hallucination, you could add your own uppercase-named file and link to it from default.css, no problem)
• it does not provide any info about the images/ subfolder to redefine one's own images