Add guide for adding custom themes

[rabail-aamir]

Description

A new markdown document named add-theme-guide.md exists within the Documentation/ folder. The guide provides instructions for adding custom themes to SplashKit Online through theme.js modifications and color.css updates and index.html changes.

Type of change

• [ ✓] Documentation (update or new)

How Has This Been Tested?

The instructions were verified to match the current project structure and implementation. The .md file was previewed in Visual Studio Code to verify the correct formatting.

Checklist

• [✓ ] My code follows the style guidelines of this project
• [✓ ] I have performed a self-review of my own code
• [✓ ] I have commented my code in hard-to-understand areas
• [✓ ] I have made corresponding changes to the documentation
• [✓ ] My changes generate no new warnings
• [✓ ] I have requested a review from ... on the Pull Request

[Oliver-Quail]

Hi @rabail-aamir,
Please can you add the other variables in the colors.css file to this documentation. There should be a few more that can be overridden.
Other than that, this is a great PR :)

[rabail-aamir]

Hi @rabail-aamir, Please can you add the other variables in the colors.css file to this documentation. There should be a few more that can be overridden. Other than that, this is a great PR :)

Yup, thank you for pointing out. All done.