Click on the profile icon in the top right corner of the application.

In the opened window, click on the "Theme" section.
Choose your preferred theme: Rain, Stars, or Ocean, by clicking on the corresponding image. Then, click the "Apply" button.

How to Choose a Different Theme Background and Adjust Theme Sound in the App?
In the same section, you can adjust the sound of the selected theme.
Below the suggested themes and the "Background music" label, you'll see a blue slider. Drag the slider on the track to set a comfortable volume level, then click the "Apply" button.

Note that the Stars theme does not have accompanying sound.

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