How can one change the theme in Power BI?

Power BI themes refer to sets of colors, fonts, and other stylistic elements that can be enforced across reports and dashboards to achieve uniformity in look and feel. Applying themes manages the appeal and readability in reports and dashboards. Power BI comes with various built-in themes and allows you the ability to create custom themes as well.
AI-written text: Go to the top ribbon and click View.

In the Themes section, click the dropdown arrow next to Themes. A menu will pop up showing various theme options.

Choose one specific theme. Apart from the inbuilt themes, if you have any specific custom themes to be brought in, this is the place.

On choosing a theme, the colors, fonts, etc., of your report will immediately reflect the theme selected.
To create or edit a custom theme, use the option for Customizing Current Theme, in the Themes drop-down menu. This will let you change colors, fonts, and styles.
Moreover, you can look at more themes that have been created by others in the Power BI community in the Theme Gallery. Open the gallery by choosing Theme Gallery from the Themes drop-down menu.

You can also explore external tools for generating themes.