Update theme override doc

[cavasinf]

Description

Add more detailed DOC about theme override

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I updated the documentation (see here)
• I agree that this code will be published under the MIT license

[kevinpapst]

@cavasinf you write "add more detailed doc" but the PR includes only the adapted scss file.
Can you share more details? Because I do not get it. Is this your set of fixes for Tabler? What does it do?

[cavasinf]

I've just added more scss color override like light backgrounds, border colors and text colors.
They were not overridden by variables colors of Tabler's default css.

I mean doc because it is not used by the Bundle. It's just an example if devs want to change default Tabler's colors.

[kevinpapst]

Okay then 😄