How to make a bottom content bar always on-screen? (ie, fixed position?)

Hey there, I have a bit of a tricky problem! 

For a client site, I'll be needing to create a full-width newsletter signup bar with a background color at the bottom of the web browser that remains visible ALL THE TIME, on top of the page content during scroll - about 100px in height, so that people can see it on every page.

See example here - this is a wordpress site, but exactly what my client wants for that bottom-of-page always-on-top newsletter call to action:

https://pippagrant.com/

For the life of me, I cannot figure out what to do with code to create this functionality! 😅 I know how to create a sticky header, but when it comes to always-visible bottom blocks, I have no clue how to set that up. Any help or direction would be so appreciated, thank you!!