Element spacing should use multiples of --default-grid-baseline

[ChristophWurst]

Steps to reproduce

1. Inspect the app DOM

Expected behavior

Spacing uses multiples of var(--default-grid-baseline)

Actual behavior

Spacing in this app uses "magic" numbers like 24px, 10px, etc

Mail app version

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Mailserver or service

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Operating system

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PHP engine version

None

Web server

None

Database

None

Additional info

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