On the global, as well as on the survey Settings/Theme Options tab, the options for images (for both logo and background), are not saved correctly. When saving, in the template_configuration table, in the database, appears the following (notice the "invalid"):
\"brandlogofile\":\"invalid:image::generalfiles::logo_auth.png\"

On the images tab (survey theme options), in the logo field appears the following (see screenshot1):
Inherit [inherited value: invalid:image:theme::files\logo.png]

As a result, no logo or background images appear, although defined, and in the nav_bar.twig, the "aSurveyInfo.options.brandlogofile" specification does work, unless the image file is explicitly specified as follows:
{{ image('./files/logo.png', aSurveyInfo.name, {"class": "logo img-responsive"}) }}

Besides, when extending a theme, while trying to define the survey theme options for the first time, the following error message appears (see screenshot2):
Error: Malformed JSON - field files_js must be either a JSON array or the string "inherit". Found "null".