The use of colour is an important part of making digital content accessible. Many visual impairments impact a person's ability to distinguish colours with low contrast. An example is using white text on a yellow background. Even for non-impaired individuals, low contrast design can cause eye strain, so using high contrast benefits all users.
Both the minimalist and pop branding themes have inbuilt high contrast. However, it is up to you to select high contrast branding colours in the branding settings. It's also important to keep in mind when you select images for the backgrounds of your blocks, as the text will sit on top of the image. Double check that your background will be accessible across all screen sizes by using the preview function.
Many third party websites can test your chosen colours to determine their level of accessibility for free. One of our favourites is https://webaim.org/resources/contrastchecker/