fix(ui): add missing override for GlobalStyle

packages/ui/src/atoms/GlobalStyle.js

 import { createGlobalStyle } from 'styled-components'
-import { th } from '@pubsweet/ui-toolkit'
+import { th, override } from '@pubsweet/ui-toolkit'
 
 const GlobalStyle = createGlobalStyle`
   body {
     color: ${th('colorText')};
   }
+  ${override('ui.GlobalStyle')};
 `
 
 export default GlobalStyle