Glossary
SharePoint theme
A SharePoint theme is the palette applied to a site — its accent colour, its neutrals and its light or dark variant. Well-built web parts read it and adopt it, which is why they look like part of the page instead of something pasted onto it.
It is the cheapest branding you will ever do on an intranet: a theme plus a logo gets you most of the way to something that looks like your organisation, without a design project.
It is also a good test when evaluating a third-party component. If it ignores the theme, or renders inside its own container with its own colours, users will read it as a bolted-on tool — and on an intranet that has a measurable effect on whether they use it.