I have the same project and if I run npm start everything is fine and the background is white as it should be but when I run npm run deploy to upload it to gh-pages the background of the page turns grey, I didn't change anything in the styles and it is the same project here I leave the photos to compare one is white background and the other gray.
What do you recommend that I do? What better way to try uploading it to netlify?
Checking your source code , you're importing your css definition (where I assume your white background is) before the material-kit libraries, which eventually overwrites the one you put:
Change the order in which you are loading your material and custom css, in general yours should always be last. In the worst case, if it still doesn't work for you, you could use
!important
to override the already defined styles: