Improve usability of tree editor by enhancing CSS styling

[ndoschek]

I suggest the following two improvements to increase usability for the tree-editor:

• The split panel handle between tree and form editor should be visible by styling one editor panel with a different background color to indicate the existence of the split panel handle (as it is commonly used in the whole Theia application).
• Input fields in the form editor using the light theme are not visible, this should be styled similar to the Theia base styling for input fields.

Missing input field visibility in light theme:

Compared to dark theme: