Touch pane of the Visuals category
Use the Touch pane of the Visuals category in VoiceOver Utility to customize the transparency of the background that dims the screen around the focus area. This pane is shown only if a trackpad is available.
To open VoiceOver Utility, press Command-F5 to turn VoiceOver on, then press VO-F8.
Note: VO represents the VoiceOver modifier.
Drag the slider toward 0 percent or 100 percent to decrease or increase the transparency of the dimmed background around the focus area.
If you increase the transparency, the background color becomes lighter and you can more easily see the rest of the screen.