[Ambele]

That's a design decision by the ReactJS. Some consider this a flaw of ReactJS. Usually all CSS stylesheets are not included globally in all pages.

Also, if you need to apply a different style, you can just include your custom stylesheet after the global or base stylesheet. As long as you use either the same selectors or more specific selectors, you will be able to override the previous styles. This is assuming the previous developer didn't make the bad design decision of using !important which usually can't be overwritten.