[SMS App] Fix message sending area from always having a white background
Summary
Replace the (deprecated and removed) reference to Kirigami.Theme.viewBackgroundColor with Kirigami.Theme.backgroundColor
This element is necessary in order to have theme hot-switching work correctly
BUG 434791
Test Plan
Before:
When using the KDE Connect SMS app in an un-modified dark theme, the message entry text box would be rendered as all white and text would correctly be rendered in white (making it un-readable).
This seems to be somehow related to a recent Kirigami version (it only happened to me after doing a distro upgrade and my ancient Kirigami installed to ~/.prefix does not reproduce the issue).
viewBackgroundColor was deprecated and apparently removed from Kirigami a version or two ago. Thus the coloring was no longer correct.
For screenshots and discussion, see bug 434791
After:
Text and background are rendered in the expected colours for the theme.