gui_qt/add_translation: fix #1569

Setting the stylesheet resets the palette colors to the system default.
For the base color, that'd mean that it'd be reset to `#ffffff`.

While setting the palette from the stylesheet could be done, it's much
simpler to instead let the formatted text inherit
the background color.

news.d/bugfix/1571.ui.md

@@ -0,0 +1 @@
+Fix "add translation" dialog ignoring the stylesheet's background color for the translation and stroke text.

plover/gui_qt/add_translation_widget.py

@@ -42,14 +42,12 @@ def __init__(self, *args, **kwargs):
 
         self._special_fmt = (
             '<span style="' +
-            'background-color:' + self.palette().base().color().name() +';' +
             'font-family:monospace;' +
             '">%s</span>'
         )
 
         self._special_fmt_bold = (
             '<span style="' +
-            'background-color:' + self.palette().base().color().name() +';' +
             'font-family:monospace;' +
             'font-weight:bold;' +
             '">%s</span>'