[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](https://bugs.kde.org/show_bug.cgi?id=434791) ### After: Text and background are rendered in the expected colours for the theme.