The themes in the Design tab provide matched sets of colors and styles for your HelpXplain projects. Changing a theme applies the theme to all the elements in your project that have not been formatted individually. The theme is previewed in the active project when you move the mouse pointer over it.
If you select a background image before selecting a theme, HelpXplain will automatically auto-generate two themes based on the colors in the background image. It's thus a good idea to select your background first, as this can provide a matching theme for the background automatically. See PSize & Background for details on setting background images.
The Customize Themes tool in the Design tab opens a dialog in which you can customize the colors, fonts and CSS styles for the theme. Each theme can store a separate set of settings and styles.
The current Xplain always stores only one edited theme, which is the theme that it uses itself. When you choose a theme the Xplain's own internal theme is overwritten with the settings of the chosen theme.
When you select a new theme this will overwrite the current theme, along with any changes you have made to that theme!