Fix ApplicationScreenshots shadow for Breeze Dark.

Summary:
If you're using Breeze Dark or probably any other dark theme, the screenshot shadow-gradient will run from white to transparent. With this patch, the shadow runs from the currently set background color to transparent.

Before:
{F5720807}

After:
{F5720808}
{F5720813}

Reviewers: apol, ngraham, #discover_software_store

Reviewed By: ngraham, #discover_software_store

Subscribers: davidedmundson, acrouthamel, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10701