What a Power BI report theme is

A Power BI report theme is a JSON file. It sets the colour palette, text defaults, and visual styles for a report. Desktop loads it once. After that, new pages and new visuals inherit the same decisions instead of being formatted by hand.

What the file contains

Named colours, data colours, foreground and background, and optional visual-level defaults. The file is theme.json. You import it in Desktop or apply it to a report in the service when the tenant allows it.

Without a theme, every card and chart has its own format pane. The next page will not match. The next author will guess.

What I use it for

One palette across a workspace of operational reports. Contrast that still works on a projected screen. A name on the file so operations know which theme is live.

BI Workbench can import an existing theme, adjust it, preview a mock page, and export. The aim is a file you can load, not a set of screenshots.