Update documentation for customizing bootstrap to use Sass

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Users should be able to import the scss files now and adjust it directly.

[mslooten]

I'd like to take a stab at this if that's OK.

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Go for it!

[vinay72]

You can Refer to this link https://getbootstrap.com/docs/4.0/getting-started/theming/

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This was done in #5211