Improve EntityModeler with Dark Theme

[sgaertner]

• Added white outlines to dark icons. Almost unchanged in Light Theme but now clearly visible in Dark Theme.
• Changed color used to indicate prototyped attributes from red to cyan. Better visible in Dark theme and looks less like an error / warning.
• Use checkboxes instead of toggles. For toggles the current state is hardly visible in Dark theme. Furthermore checkboxes are in general more verbose.