[theming] input area not visible on light themes when in the main area

[vince-fugnitto]

Description

The --theia-input-background color used for input areas is not visible on light themes when widgets are present in the main area. The color works well for the sidebars but is hardly visible otherwise.

For example (the keyboard shortcuts widget):
Notice the input area with the add search term.

Main Area

Sidebar (Activity Bar)

Steps to Reproduce

1. prerequisite: VS Code like theming (colors and icons) #6475
2. start the application
3. navigate to the keyboard shortcuts widget
4. switch to a light theme
5. notice the input is hardly visible (dock the widget to the side)

[akosyakov]

Background seems to be fine, it is the same in VS Code:

But input borders are missing. We should check which colors are used in VS Code for the keybinidng widget and apply them as well.

[akosyakov]

It's turned out to be hardcoded in css styles for VS Code: https://github.com/microsoft/vscode/blob/ff5f581425da6230b6f9216ecf19abf6c9d285a6/src/vs/workbench/contrib/preferences/browser/media/preferences.css#L127

will do the same