Hi @cininik966917w, welcome to Thunkable! 
Be sure to check out our posts about How to ask Great Questions v2.0 and our Community Guidelines as you get started.
There are a few previous discussions about this in the Community already. We encourage you to search, as it may make your answer easier to find. I will link to this discussion below which has an answer that is still working for testing for and creating a dark mode for your app.
Please let us know if we can help with anything else!