Sunburst charts feature 2 titles in the center of the visualization. The first title (the measure title) displays the total value of the measure chip in the Values drop zone; the second title (the caption title) displays the measure's caption.
The formatting of the Sunburst titles is automatically determined by the given report theme. However, the formatting can be customized by opening the Sunburst Titles dialog from the Component ribbon:
Sunburst Titles Dialog
Auto: automatically use the formatting of the given report theme.
Measure: change the font type, size, color, and emphasis of the measure title (the value displayed in the middle of the chart).
Caption: change the font type, size, color, and emphasis of the caption title (the measure caption displayed beneath the value).
In this example, the font color, size, and emphasis was adjusted for both the measure and caption titles: