
Hi @Paul Moloney ,
1.If you want to customize your website theme, you can define a new custom theme through an online theme generator tool. Select the main color, text color and background color, export the theme, then copy the PowerShell code from the export theme option to execute it through a power shell script. Or themes are defined in a JSON schema that stores color settings and associated metadata for each theme.
Reference: SharePoint site theming
2.Create site templates to provide lists, themes, layouts or custom actions.
Reference: Get started creating site templates
3.If you're a designer, you might be able to use the SharePoint site publishing capabilities to build, customize, and maintain publishing sites that meet specific business needs.
Reference: Use Design Manager to implement a design
Deploy a custom theme in SharePoint
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