Styleguide - not a UI kit
As a UX designer many different tasks must be managed and creating a styleguide could be one of them. It is a tool and great help for yourself and your team to keep the design consistent. This is not the same as a UI kit to me, this is the style framework that does not include the icon library. I usually don’t create icons, I use existing UI kits in case there is not one already in the project I’m working on.
What to include in a styleguide?
This may differ between projects but I usually define the following:
Heading structure (H1-H5): font type, font size, and font style
Paragraph text
Help text
Colors including HEX code, name and short explanation how to use
Links: external and internal
CTA: primary, secondary, hover state, disabled state