ColorTheme Wizard doesn't display the logo when creating a new ColorTheme or visualizing a theme that has a white header background

Since 3.4, the DefaultColorTheme overrides colibri standard logo with its own: a white XWiki logo.
When displaying a theme in the Wizard, the logo image location is computed using $logourl (from global.vm). This means the displayed logo image is the one from DefaultColorTheme, which results in displaying a white logo on a white background (value taken from the ColorThemeTemplate).