Creating a styleguide is a great way to maintain consistency in your product even if you’re not working on a greenfield product. It’s a great way to document the typography styles, colour palette and input styles. It will not only help with your own sanity, but it’s a great way to bridge the gap between designers and developers as they can refer back to this to sense check what they’ve built.
You can use Sketch to create a styleguide and it’ll do a lot of the work for you by pulling in your colours and fonts. Another option is to use a more specific tool such as Zeplin (https://zeplin.io). If you also use this for design hand off, then this may be a tool your developers feel more comfortable using.