Checkboxes should use theme colours

Description

Checkboxes in the RHS don't use theme colors currently.

They should use:

QA Test Steps

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Mana

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Assignee

Jesse Hallam

QA Assignee

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Reporter

Christopher Speller

Epic Link

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Fix versions

Mattermost Team

Workflows

Sprint

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Labels

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QA Testing Areas

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GitHub Issue

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Components

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Severity

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