Changed default plot background to white.
Changed default plot background to white.

paulromano/openmc,lilulu/openmc,johnnyliu27/openmc,wbinventor/openmc,paulromano/openmc,nhorelik/openmc,shikhar413/openmc,wbinventor/openmc,shikhar413/openmc,samuelshaner/openmc,bhermanmit/openmc,smharper/openmc,paulromano/openmc,shikhar413/openmc,johnnyliu27/openmc,kellyrowland/openmc,kellyrowland/openmc,mit-crpg/openmc,bhermanmit/cdash,lilulu/openmc,nhorelik/openmc,liangjg/openmc,paulromano/openmc,keadyk/openmc_mg_prepush,lilulu/openmc,samuelshaner/openmc,liangjg/openmc,mit-crpg/openmc,shikhar413/openmc,smharper/openmc,mit-crpg/openmc,bhermanmit/openmc,sxds/opemmc,johnnyliu27/openmc,amandalund/openmc,walshjon/openmc,samuelshaner/openmc,wbinventor/openmc,amandalund/openmc,amandalund/openmc,shenqicang/openmc,liangjg/openmc,wbinventor/openmc,mjlong/openmc,amandalund/openmc,smharper/openmc,keadyk/openmc_mg_prepush,walshjon/openmc,sxds/opemmc,liangjg/openmc,walshjon/openmc,samuelshaner/openmc,keadyk/openmc_mg_prepush,mjlong/openmc,johnnyliu27/openmc,shenqicang/openmc,smharper/openmc,walshjon/openmc,mit-crpg/openmc