Implemented concurrent CSS builds for faster builds

[jackdomleo7]

Description

• Noticed an opportunity to improve the build time of the CSS files by building them concurrently

Why is this important?

• A greener project

Covered test cases

• N/A

Did you test on all major browsers?

I couldn't test Safari because I don't have access to an Apple device, however N/A anyway

• Chrome
• Firefox
• Edge
• Safari

Other details

N/A

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T&Cs

• I confirm I have read and understand the contributing guidelines
• I understand the work in this pull request will not be released straight away and will appear in a future release (if approved)
• I confirm the work in this pull request is true and valid to the best of my knowledge
• I have updated the README, features.md and codes.md files where applicable