Reset color pairs when new theme is loaded

We only need the colour pairs initialized that the theme actually uses.
It's otherwise possible that we run over the max value of initialzed
pairs.

src/config/theme.c

@@ -176,6 +176,8 @@ theme_exists(const char *const theme_name)
 gboolean
 theme_load(const char *const theme_name)
 {
+    color_pair_cache_reset();
+
     if (_theme_load_file(theme_name)) {
         _load_preferences();
         return TRUE;