[2.3] [improvement] Swatches LESS rewrite

[Karlasa]

Rewrote LESS code to suit better with Magento LESS mixins and variables.

Fixed Issues (if relevant)

File was missing LESS variables

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-cicd2]

All committers have signed the CLA.

[VladimirZaets]

Hi, @Karlasa, I took your PR into processing, thank you for collaboration.

[magento-engcom-team]

Hi @Karlasa. Thank you for your contribution.
We will aim to release these changes as part of 2.3.0.
Please check the release notes for final confirmation.