Toolbars and notifications are not properly colored

[fcollonval]

Description

Some elements like toolbars and notifications are not properly colored.

The reason is: those elements use their own CSS property that fallback to the JupyterLab theme CSS property. Unfortunately as we set the CSS properties on the body element, those properties are not updated and keep the value of the JupyterLab theme CSS property initially loaded.

Possible solutions:

• overwrite those CSS properties -> not very robust as their list may change in future version
• properly inject a CSS style sheet with custom values