There is a file inside your installation path where you can customize your base layer/s and servers: WEB-INF/config-gui.xml. It contains some instruction about this configuration.
You can define base layers for both the mapSearch (the smaller one) and the mapViewer.
Here's an example from a working Geonetwork config:
<mapSearch options="{ projection: 'EPSG:4326', units: 'degrees', restrictedExtent: new OpenLayers.Bounds(-63,-27,-57,-22), }">
<layers>
<layer server="http://wms.ign.gob.ar/geoserver/wms?" tocName="Capa Base IGN" params="{ layers: 'IGN:limite_politico_administrativo_lim,IGN:localidad', transparent: 'true', format: 'image/png'}" options="{isBaseLayer: true}" />
</layers>
</mapSearch>
<mapViewer options="{ projection: 'EPSG:4326', units: 'degrees', restrictedExtent: new OpenLayers.Bounds(-63,-27,-57,-22) }">
<layers>
<layer server="http://wms.ign.gob.ar/geoserver/wms?" tocName="Capa Base IGN" params="{ layers: 'IGN:limite_politico_administrativo_lim,IGN:localidad', transparent: 'true', format: 'image/png' }" options="{isBaseLayer: true}"/>
</layers>
<scales values="[7500000, 5000000, 2500000, 1000000, 750000, 500000, 250000, 100000, 75000, 50000, 25000, 10000, 7500, 5000, 2500, 1000]"/>
<!--scales values="[]" /-->
<proj>
<crs code="EPSG:4326" default="1" name="WGS84 (lat/lon)"/>
</proj>
<servers>
<server name="NASA JPL OneEarth Web Mapping Server (WMS)" url="http://wms.jpl.nasa.gov/wms.cgi?"/>
<server name="NASA Earth Observations (NEO) WMS" url="http://neowms.sci.gsfc.nasa.gov/wms/wms?"/>
<server name="DEMIS World Map Server" url="http://www2.demis.nl/mapserver/wms.asp?"/>
<server name="IDEF servicio WMS" url="http://idef.formosa.gob.ar/servicios/wms?"/>
</servers>
</mapViewer>
Hope this helps.
Regards
Cristian