Step by Step : 

Step 1— Go to Branding

• From the left menu, click on Branding under Settings & Customization.

• Select Review widgets, and click on the setup widget.

Step 2 — Navigate the review widget

• From the Review widgets page, 

• Find the Review widget and click on customize.

Step 3  — Set Widget Colours

• Click on customize widget colours to match your brand theme.
  (light, dark, custom)