Add setting for the webpage bg color to use.

Fixes #719.

CHANGELOG.asciidoc

@@ -37,6 +37,7 @@ Added
 - New flag `-d`/`--detach` for `:spawn` to detach the spawned process so it's not closed when qutebrowser is.
 - New (hidden) command `:follow-selected` (bound to `Enter`/`Ctrl-Enter` by default) to follow the link which is currently selected (e.g. after searching via `/`).
 - New setting `ui -> modal-js-dialog` to use the standard modal dialogs for javascript questions instead of using the statusbar.
+- New setting `colors -> webpage.bg` to set the background color to use for websites which don't set one.
 
 Changed
 ~~~~~~~

qutebrowser/browser/webview.py

@@ -24,6 +24,7 @@ import itertools
 import functools
 
 from PyQt5.QtCore import pyqtSignal, pyqtSlot, Qt, QTimer, QUrl
+from PyQt5.QtGui import QPalette
 from PyQt5.QtWidgets import QApplication, QStyleFactory
 from PyQt5.QtWebKit import QWebSettings
 from PyQt5.QtWebKitWidgets import QWebView, QWebPage
@@ -108,6 +109,7 @@ class WebView(QWebView):
         self.search_flags = 0
         self.selection_enabled = False
         self.init_neighborlist()
+        self._set_bg_color()
         cfg = objreg.get('config')
         cfg.changed.connect(self.init_neighborlist)
         # For some reason, this signal doesn't get disconnected automatically
@@ -181,6 +183,15 @@ class WebView(QWebView):
         self.load_status = val
         self.load_status_changed.emit(val.name)
 
+    def _set_bg_color(self):
+        """Set the webpage background color as configured."""
+        col = config.get('colors', 'webpage.bg')
+        palette = self.palette()
+        if col is None:
+            col = self.style().standardPalette().color(QPalette.Base)
+        palette.setColor(QPalette.Base, col)
+        self.setPalette(palette)
+
     @pyqtSlot(str, str)
     def on_config_changed(self, section, option):
         """Reinitialize the zoom neighborlist if related config changed."""
@@ -195,6 +206,8 @@ class WebView(QWebView):
                 self.setContextMenuPolicy(Qt.PreventContextMenu)
             else:
                 self.setContextMenuPolicy(Qt.DefaultContextMenu)
+        elif section == 'colors' and option == 'webpage.bg':
+            self._set_bg_color()
 
     def init_neighborlist(self):
         """Initialize the _zoom neighborlist."""

qutebrowser/config/configdata.py

@@ -959,6 +959,11 @@ def data(readonly=False):
              SettingValue(typ.QtColor(), 'red'),
              "Background color for downloads with errors."),
 
+            ('webpage.bg',
+             SettingValue(typ.QtColor(none_ok=True), 'white'),
+             "Background color for webpages if unset (or empty to use the "
+             "theme's color)"),
+
             readonly=readonly
         )),