Since no one answered to my question (Material UI Button loses Styling after page refresh) I'm asking again, this time including a CodeSandbox: https://codesandbox.io/s/bold-resonance-8c8pz?file=/src/components/style/Login-styling.js
Steps to reproduce problem:
1-Change a value in the "Login.styling.js" for the button (just to make react update the page) and now the button is the color you set.
2-Refresh the page (the button will not follow the style anymore)