To edit an existing theme in Theme Designer, perform the following steps in Canvas Studio.

On the home page of Canvas Studio, click Theme Designer on the left pane.
The existing themes appear. Right-click the theme which you want to edit and click the Edit option.
The Theme Designer page appears. You can edit the themes of the fields, for which you wish to apply changes. Click icon of the respective fields, for which you wish to modify the themes.
If you wish to create a clone for the theme, click Clone option.
A new Theme Designer page appears and you have to type the relevant values in the relevant fields.
After you have completed the proceedings, click Submit to save the changes.

The following screen shot serves as a good illustration of the output in the functional application, Modelhouse, showcasing the applied theme on the menu layout.

The following screen shot from the functional application, Modelhouse, showcases the modified background color of the app.

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