Click on the Settings Cog in the utility bar.
Select Accessibility Checker from the Analytics + Tools menu.
Colors that do not pass the accessibility test will appear in the Fail section.
Click the Swatch or name of the Color Style to open the color in branding.
Adjust the colors as necessary to increase the contrast ratio, then click Apply.
For more information on contrast ratios and color tests, see Synthesis Accessibility Checker: Contrast + Color Tests.