The 0.4.3 version of sphinx_rtd_theme supports the possiblity to change
background of the search area. This change modifies the background to
white to make it inline with the ONAP logo.
Issue-ID: DOC-402
Change-Id: I5156cf0ed146e12d9dbd2b5685a7ff563a88f6a2 Signed-off-by: Gergely Csatari <gergely.csatari@nokia.com>