plugins: text viewer: use theme colors & backdrop
When Night Mode was added to Text Viewer
in ab1b67f, the regular mode was changed to
black and white instead of using theme colors.
This looks out of place in custom themes when
the statusbar is enabled, i.e. using the default
Text Viewer setting. Only switch to black and
white when statusbar is disabled, which also
disables the backdrop.
Change-Id: I3ff5e1c42de11ca58640e91bcb315ce7804fadcd