Enabling dark mode before starting ubiquity causes UI bugs on Ubiquity KDE-UI mode

[Joshua Peisach]

If the user sets the global theme to Dark (Breeze-Dark) before launching ubiquity, then a good chunk of the text (especially on Wireless page) will be gone.

The text, some of the UI colors maybe, need to be fixed in total.

On an 11.5 in macBook from about 3y ago

EDIT: I did research and these bugs only occur on launching ubiquity with kde-ui and dark mode.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: ubiquity 20.04.9
ProcVersionSignature: Ubuntu 5.4.0-21.25-generic 5.4.27
Uname: Linux 5.4.0-21-generic x86_64
ApportVersion: 2.20.11-0ubuntu22
Architecture: amd64
CasperVersion: 1.442
CurrentDesktop: KDE
Date: Mon Apr 6 14:42:40 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/kubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Kubuntu 20.04 LTS "Focal Fossa" - Beta amd64 (20200406)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Ubuntu QA Website (ubuntuqa)]

This bug has been reported on the Ubuntu ISO testing tracker.

A list of all reports related to this bug can be found here:
http://iso.qa.ubuntu.com/qatracker/reports/bugs/1871131

[Nafiur Rahman Khadem (shafinkhadem)]

A temporary fix: edit /usr/share/ubiquity/qt/style.qss to replace

```
#setup_page,
#footer_frame {
    background-image: url(/usr/share/ubiquity/qt/images/main.png);
}
```

to

```
#setup_page,
#footer_frame {
    background-color: #777;
}
```

This will match both light and dark theme fairly well.