From Xojo Documentation
Use the Style Editor to create Styles for your web applications. It provides a simple unified interface for specifying all aspects of a control’s appearance. To add a Style to your project, select “Web Style” from the Insert button on the toolbar or the Insert menu. When you click on the Style, you see the Style Editor in its initial blank state. It contains controls for specifying when the style is used (Normal, Hover, Pressed and Visited) and a button to add Style Properties.
Use the Add Property button to add a new Style property. There are a variety of properties to choose from, dealing with Text/Fonts, Borders, Shadows, Padding, Corner Radii, Opacity and the Background. Choose the property you want to add and then use the editor to modify the setting as needed.
UserGuide:Web Styles topic