Fix download page background color

Summary:
Dear upstream maintainer,

while updating the artikulate package for Debian, I noticed that the current code doesn’t correctly handle dark themes when displaying the list of downloads.
When using breeze dark for example the name of the items are light grey on a white background.

Test Plan:

• Select breeze (light) application theme
• Start Artikulate and go to the download page
• Check that the text of the items to download is readable.
• Swtich to the breeze dark application theme
• Check that the text of the items to download is still readable.

Reviewers: aacid

Reviewed By: aacid

Subscribers: aacid, kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D26804