fix: Add white background body element so Onboarding background consistent in taskbar app

* Note: To quickly see the changes force the Onboarding component (displaying Terms & Conditions screen) to appear by changing
`if (isFirstRun) {` to `if (true) {` in `fether-react/src/App/App.js. The screen should be consistent when you run `yarn taskbar` and `yarn start`

* Add eggshell background colour to `body` element  otherwise `yarn taskbar` app has grey background (even though
running as normal `yarn start` or `yarn electron` app has white background from manifest)