Site styling and templates
A theme describes the colors, fonts, section margins, and other design elements of a Power Pages site. Each site is provisioned with a theme defined by the selected template. By using Styling workspace elements, you can customize the theme to align the site appearance with design requirements such as specific corporate branding.
Styling
Choose a theme from several available presets and then apply customizations. You see any customization immediately on the page, making it easy to work with the basic styling.

Custom CSS
For more complex styling requirements, Power Pages design studio allows makers to upload custom CSS files.

Once you upload the custom CSS file, it applies to all themes, and the page canvas reflects the applicable styles. You can upload multiple CSS files and set their order as required. For more information about custom CSS, see Manage CSS files in Power Pages in product documentation.
You can also apply styling to individual pages by using Visual Studio Code for the Web editor. When you open a page in the editor, three files load: HTML, CSS, and JavaScript. Any changes in the CSS file apply to the selected page only.
Templates
For complex sites, standard layouts available in Power Pages design studio might not be sufficient. For these scenarios, Power Pages supports creating pages by using custom layouts.
Because Power Pages is built on the foundation of Power Apps portals, it can use page templates created by makers and pro developers in Portal Management app. When you define a new Page Template, it becomes available in the design studio as a custom layout.

For detailed information on how to create custom page templates, see Create and manage page templates and Store source content by using web templates in the Power Apps documentation.