refactor bpk-component-loading-button styles

[mungodewar]

Remember to include the following changes:

• [ ] Ensure the PR title includes the name of the component you are changing so it's clear in the release notes for consumers of the changes in the version e.g [KOA-123][BpkButton] Updating the colour
• [ ] README.md (If you have created a new component)
• [ ] Component README.md
• [ ] Tests
• [ ] Storybook examples created/updated
• [ ] Type declaration (.d.ts) files updated
• [ ] For breaking changes or deprecating components/properties, migration guides added to the description of the PR. If the guide has large changes, consider creating a new Markdown page inside the component's docs folder and link it here