Step 1: Navigate to Appearance from your admin dashboard then click Customize. Next, hit Header > Style.
Step 2: Here you can customize your header style by choosing font, size, weight and color.
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.
Step 3: After customizing, do not forget to click Publish button to save your changes.