[preview] fix transparent preview widget background

[vince-fugnitto]

What it does

Fixes #6194

• Fixes an issue where widgets present in the main area had transparent backgrounds which interfered with custom branding. With the change, widgets now have default background colors.
• Adjusts the styling of the dockpanel to be more consistent with VSCode

Before (master)	After (pr)

How to test

I made the following changes locally in the index.css so I can visualize the changes:

#theia-main-content-panel {
  background-image: url('https://assets.webiconspng.com/uploads/2017/09/Simpsons-PNG-Image-19701.png');
  background-position: center center;
  background-repeat: no-repeat;
  background-size: 50%;
}

1. with the local changes, open a workspace (ex: 'theia')
2. open a README.md or any markdown file
3. click on preview in the markdown's tabbar toolbar
4. confirm that the contributed image is not visible
5. open any widget in the main area (ex: explorer, scm, getting-started, search) and ensure that styling is correct and the background image does not display

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

Signed-off-by: Vincent Fugnitto [email protected]

[lmcbout]

Can still see Homer in the preferences settings and the Getting started page.

[vince-fugnitto]

Can still see Homer in the preferences settings and the Getting started page.

I'll fix them as well, I believe widgets should have default backgrounds and not be transparent.

[vince-fugnitto]

@lmcbout the code has been updated to give a background-color to widgets present in the main content area. You can further test by dnd a widget from the sidebar into the main area
(ex: explorer, scm, ...)

[lmcbout]

LGTM
Homer is Gone :(
Tested with DND, Getting started, preferences: OK
Tested on Firefox and Chrome

[vince-fugnitto]

It looks like this bug is back :(
@akosyakov is it a bug or what is by design to remove the behavior with the theming pull-request?

[akosyakov]

@vince-fugnitto yes, it is a bug, we don't have such colors, please send another PR to apply suitable VS Code colors (not by value, but by meaning)

[GNSubrahmanyam]

@vince-fugnitto

This issue is still there

Steps to reproduce:

1. go the preferences

Issue reference: #8444