In order to access this feature, you need to navigate to the theme customizer page where the settings below are available.

WordPress theme customizer has the built-in styles customization function which will help you to set some custom codes as per your requirement. The theme fully supports it. We recommend this feature for experienced users who are familiar with the CSS knowledge, otherwise, you might break the website layout. To add your custom codes navigate to the Additional CSS section of the customizer.

Code Example Link

Your desired Additional CSS which will modify widgets title font color and size might look for example like this:

.widget-title {
    color: #18be95;
    font-size: 1.5rem;
}