Add explicit color flag for reverse video mode

Reverse video mode is useful when using the CCG/XEP-0392 palette.
Since we don’t know the true background color and the CCG palette
is (presumably) made for a specific background color (range), it
can currently only be used as foreground colour against a `-1`
background. Reverse video mode also allows it to be used as
background colour (with the normal background colour as foreground
colour).

poezio/theming.py

@@ -487,6 +487,8 @@ def to_curses_attr(
                 curses, 'A_ITALIC') else curses.A_REVERSE)
         if 'a' in additional_val:
             curses_pair = curses_pair | curses.A_BLINK
+        if 'r' in additional_val:
+            curses_pair = curses_pair | curses.A_REVERSE
     return curses_pair