Goal: To define content (phone number, address etc) in the admin area that can be used (called upon) in a theme.
Context: I am making a theme for a multi-site where every site will look the same, but have different content. The user should be able to easily add basic information in the admin area, preferably accessible through the left menu. The data fields will be predefined. It is important that each piece of data can be fetch independently.
My initial thoughts: Making a simple plugin where the user can enter the information. I was thinking of putting it in the functions.php file, but was not sure what would happen to the data if I made a new theme in the future.
Are there other obvious ways of accomplishing this that I haven't thought of? Any suggestions to how I could solve this in a future proof way?
Also, recommendations for good resources on the topic is appreciated!