[Task](ui): Adjust AppShell spacing

[guoda-puidokaite]

Task Description

Now

Improvements

Add space between SideNav and content and remove space between PageHeader and content.

PageFooter should be in "juno-content-container" and it's background should be transparent to take on containers colour:

Sub-tasks

• Sub-task 1
• Sub-task 2
• Sub-task 3

Related Issues

Additional Context