MOBI Output: Fix invalid color specification as plain numbers causing conversion to fail. Fixes #1933797 [ebook-convert fails with TypeError: Argument must be bytes or unicode, got 'float'](https://bugs.launchpad.net/calibre/+bug/1933797)

More generally have the conversion pipeline validate values for color
just as it does for background-color

src/calibre/ebooks/mobi/utils.py

@@ -640,7 +640,7 @@ def is_guide_ref_start(ref):
 def convert_color_for_font_tag(val):
     rgba = parse_color_string(unicode_type(val or ''))
     if rgba is None or rgba == 'currentColor':
-        return val
+        return str(val)
     clamp = lambda x: min(x, max(0, x), 1)
     rgb = map(clamp, rgba[:3])
     return '#' + ''.join(map(lambda x:'%02x' % int(x * 255), rgb))

src/calibre/ebooks/oeb/stylizer.py

@@ -29,6 +29,12 @@ css_parser_log.setLevel(logging.WARN)
 _html_css_stylesheet = None
 
 
+def validate_color(col):
+    return cssprofiles.validateWithProfile('color',
+                col,
+                profiles=[profiles.Profiles.CSS_LEVEL_2])[1]
+
+
 def html_css_stylesheet():
     global _html_css_stylesheet
     if _html_css_stylesheet is None:
@@ -414,6 +420,7 @@ class Style(object):
         self._height = None
         self._lineHeight = None
         self._bgcolor = None
+        self._fgcolor = None
         self._pseudo_classes = {}
         stylizer._styles[element] = self
 
@@ -467,9 +474,7 @@ class Style(object):
         return self._unit_convert(self._get(name))
 
     def _get(self, name):
-        result = None
-        if name in self._style:
-            result = self._style[name]
+        result = self._style.get(name, None)
         if (result == 'inherit' or (result is None and name in INHERITED and self._has_parent())):
             stylizer = self._stylizer
             result = stylizer.style(self._element.getparent())._get(name)
@@ -491,6 +496,16 @@ class Style(object):
     def pt_to_px(self, value):
         return (self._profile.dpi / 72) * value
 
+    @property
+    def color(self):
+        if self._fgcolor is None:
+            val = self._get('color')
+            if val and validate_color(val):
+                self._fgcolor = val
+            else:
+                self._fgcolor = DEFAULTS['color']
+        return self._fgcolor
+
     @property
     def backgroundColor(self):
         '''
@@ -499,11 +514,6 @@ class Style(object):
         are not used. None is returned if no background color is set.
         '''
 
-        def validate_color(col):
-            return cssprofiles.validateWithProfile('color',
-                        col,
-                        profiles=[profiles.Profiles.CSS_LEVEL_2])[1]
-
         if self._bgcolor is None:
             col = None
             val = self._style.get('background-color', None)