Step 1: Navigate to Appearance from your admin dashboard then click Customize. Next, hit Header. You can see 4 sections to customize.
Step 2: Choose the section you want to customize.
Choose Layout to edit the Header Type and the Search box.
Choose Spacing, Border to control the general appearance.
Choose Background to edit the color.
Step 3: After customizing, do not forget to click Publish button to save your changes.
Note: If you have imported the demo, you will not be able to see the changes you made because the page already had a fixed header style.