Align view container styling with VSCode

[msujew]

What it does

Closes #10815

How to test

1. Compare the view container styling with VSCode (in particular title font weight and border styling)
2. Try this with different themes.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

The changes look much better now 👍 I just noticed one oddity:

1. start the application using theia as a workspace
2. add open editors, folder, and npm scripts as tree-view parts
3. hide open editors - the folder part has a top border

[msujew]

@vince-fugnitto You're right, I can reproduce that as well. This cannot be easily handled just with CSS, because the :nth-child(of <selector> syntax is only supported by Safari :/

I'll see how this can be addressed.

[msujew]

@vince-fugnitto The final solution is not as clean as previously, but there is no nice way to filter for the first visible view container part using purely CSS.

[vince-fugnitto]

The changes work well for me, and I confirmed the bug I noticed earlier is fixed 👍