[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.
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