Add option to unset colors provided in HTML mails for preview into Preferences

It's located in Edit->Preferences->Mail Preferences->HTML Messages->Unset
colors provided in HTML mails in message preview.

Related to https://gitlab.gnome.org/GNOME/evolution/issues/695

data/webkit/e-web-view.js

@@ -1069,10 +1069,10 @@ Evo.unsetHTMLColors = function(doc)
 				continue;
 
 			if (rule.style.color)
-				rule.style.color = "inherit";
+				rule.style.removeProperty("color");
 
 			if (rule.style.backgroundColor)
-				rule.style.backgroundColor = "inherit";
+				rule.style.removeProperty("background-color");
 		}
 	}
 
@@ -1086,10 +1086,13 @@ Evo.unsetHTMLColors = function(doc)
 		if (elem.tagName != "HTML" && elem.tagName != "IFRAME" && elem.tagName != "INPUT" && elem.tagName != "BUTTON" && elem.tagName != "IMG") {
 			if (elem.style) {
 				if (elem.style.color)
-					elem.style.color = "inherit";
+					elem.style.removeProperty("color");
 
 				if (elem.style.backgroundColor)
-					elem.style.backgroundColor = "inherit";
+					elem.style.removeProperty("background-color");
+
+				if (!elem.style.length)
+					elem.removeAttribute("style");
 			}
 
 			elem.removeAttribute("color");

src/mail/e-mail-display.c

@@ -2108,12 +2108,13 @@ mail_display_drag_data_get (GtkWidget *widget,
 	g_free (uri);
 }
 
-static void
+static gboolean
-e_mail_display_test_change_and_update_fonts_cb (EMailDisplay *mail_display,
+e_mail_display_test_key_changed (EMailDisplay *mail_display,
-						const gchar *key,
+				 const gchar *key,
-						GSettings *settings)
+				 GSettings *settings)
 {
 	GVariant *new_value, *old_value;
+	gboolean changed = FALSE;
 
 	new_value = g_settings_get_value (settings, key);
 	old_value = g_hash_table_lookup (mail_display->priv->old_settings, key);
@@ -2124,10 +2125,30 @@ e_mail_display_test_change_and_update_fonts_cb (EMailDisplay *mail_display,
 		else
 			g_hash_table_remove (mail_display->priv->old_settings, key);
 
-		e_web_view_update_fonts (E_WEB_VIEW (mail_display));
+		changed = TRUE;
 	} else if (new_value) {
 		g_variant_unref (new_value);
 	}
+
+	return changed;
+}
+
+static void
+e_mail_display_test_change_and_update_fonts_cb (EMailDisplay *mail_display,
+						const gchar *key,
+						GSettings *settings)
+{
+	if (e_mail_display_test_key_changed (mail_display, key, settings))
+		e_web_view_update_fonts (E_WEB_VIEW (mail_display));
+}
+
+static void
+e_mail_display_test_change_and_reload_cb (EMailDisplay *mail_display,
+					  const gchar *key,
+					  GSettings *settings)
+{
+	if (e_mail_display_test_key_changed (mail_display, key, settings))
+		e_mail_display_reload (mail_display);
 }
 
 static void
@@ -2408,6 +2429,9 @@ e_mail_display_init (EMailDisplay *display)
 	g_signal_connect_swapped (
 		display->priv->settings , "changed::use-custom-font",
 		G_CALLBACK (e_mail_display_test_change_and_update_fonts_cb), display);
+	g_signal_connect_swapped (
+		display->priv->settings , "changed::preview-unset-html-colors",
+		G_CALLBACK (e_mail_display_test_change_and_reload_cb), display);
 
 	g_signal_connect (
 		display, "load-changed",

src/mail/mail-config.ui

@@ -2407,6 +2407,22 @@
                         <property name="position">0</property>
                       </packing>
                     </child>
+                    <child>
+                      <object class="GtkCheckButton" id="chkUnsetHTMLColors">
+                        <property name="label" translatable="yes">Unset colors provided in HTML mails in message previe_w</property>
+                        <property name="visible">True</property>
+                        <property name="can_focus">True</property>
+                        <property name="receives_default">False</property>
+                        <property name="use_underline">True</property>
+                        <property name="xalign">0.5</property>
+                        <property name="draw_indicator">True</property>
+                      </object>
+                      <packing>
+                        <property name="expand">False</property>
+                        <property name="fill">False</property>
+                        <property name="position">1</property>
+                      </packing>
+                    </child>
                     <child>
                       <object class="GtkCheckButton" id="chkPromptWantHTML">
                         <property name="label" translatable="yes">_Prompt on sending HTML mail to contacts that do not want them</property>
@@ -2420,7 +2436,7 @@
                       <packing>
                         <property name="expand">False</property>
                         <property name="fill">False</property>
-                        <property name="position">1</property>
+                        <property name="position">2</property>
                       </packing>
                     </child>
                   </object>

src/modules/mail/em-mailer-prefs.c

@@ -1786,6 +1786,12 @@ em_mailer_prefs_construct (EMMailerPrefs *prefs,
 		widget, "active",
 		G_SETTINGS_BIND_DEFAULT);
 
+	widget = e_builder_get_widget (prefs->priv->builder, "chkUnsetHTMLColors");
+	g_settings_bind (
+		settings, "preview-unset-html-colors",
+		widget, "active",
+		G_SETTINGS_BIND_DEFAULT);
+
 	widget = e_builder_get_widget (prefs->priv->builder, "chkPromptWantHTML");
 	g_settings_bind (
 		settings, "prompt-on-unwanted-html",