Background image bug for project wizard dialog

# Issue
Some of the classes relating to elements in the new project wizard dialog (File > New Project...) have a `background` property in their CSS that makes the section of the dialog unreadable. Specifically the following classes:

- `.GEL-OVUBJ2`, the back button
- `.GEL-OVUBL2`, the main content area of the dialog

# Screenshots of problem
File > New Project ... > New Directory > New Project

File > Existing Directory

File > Version Control

# Solution
Override the `background` property of the two CSS classes, `.GEL-OVUBJ2` and `.GEL-OVUBL2`. An override is also needed when the back button is being pressed, i.e. on `.GEL-OVUBJ2:focus`. The back button override likely also needs a border/outline (as per the cancel button in the bottom right of the dialogs).

Dialog as expected (background property of CSS classes disabled via the chromium developer tools inspector)