How can I choose custom colors and a logo to personalize our handbook?

To do this, you must be an administrator in the handbook. Go to the handbook where you want to change the homepage image and enter edit mode. You can do this by clicking on the round button with three dots and selecting "Edit". 

Next, click on "Settings" in the blue bar at the top. Here, you can choose the color of the top bar and its text color. You can also select the background color and the color of the buttons in the folder structure when they are active. At the bottom, you can upload a custom logo. On the right side, there is a preview image that shows in real time how the changes will look once you save them.