Add clear documentation for creating custom themes (fixes #3106)

[konaspandana017]

This update adds a new THEME_GUIDE.md file to clearly explain how to create and structure custom themes in Chamilo LMS.

Changes made:

Added clear folder structure for CSS placement (app/Resources/public/css/themes/{theme}/ and web/css/{theme}/).

Mentioned naming conventions and duplication note for Chamilo 1.9+.

Linked to official Chamilo developer documentation for further details.

Related to issue #3106.

[ywarnier]

Hi @konaspandana017
Thank you for your contribution.
Without wanting to discourage you, you have sent this PR to the wrong branch, as the THEME_GUIDE.md you added explains how to add your own theme in Chamilo 1.11.x, which is another branch. The master branch is for the development of the next big version, Chamilo 2.0, which has a different files structure for themes.

If you want to resubmit that file in the 1.11.x, I'd be happy to analyse it in more detail and possibly merge it, but if you want this to document v2.0, you will need to apply a few changes to your .md file. Let us know what your intention is.