Use the live preview
Edit the greeting, launcher label, colours, and visible identity while watching the preview. The preview is the quickest way to catch unreadable combinations before saving.
Keep it accessible
Choose colours with clear contrast and labels that describe what opens. Do not rely on colour alone to explain whether support is available.
Check the real site
Save the design, then reload a page where the widget is installed. Browser caching or site CSS can make the live result differ from an unsaved preview.