Power BI Report Themes is a feature that allows you to apply multiple settings to a report at the same time and share them via a single JSON file. Check out this page for more information: Report Themes with Custom Visuals
Assuming you are setting up a custom JSON theme file (as described here), the customizable properties of Bullet Chart are:
| Property | Theme Color |
|---|---|
| Category Axis Color | foreground |
| Value Axis Color | foreground |
| Data Default Color | dataColors (first color) |
| Legend Color | foreground |
| Markers Color | foreground |