Setting Custom Background Colors and Pattern Fills
The default background (what appears to be the oceans in the example below) can be modified via the Data Frame Properties.
Right-click / Properties to open the Data Frame Properties sheet:
Click on the Frame tab. Under Background, choose a color from the drop-down list:
For a simple color fill, click OK...
OR: For more control over the background click on the Style Properties button:
Choose a different color, change to outline properties, etc.
For even more control, click on Change Symbol:
Choose a new symbol, optionally with a pattern fill...
OR: For even more control, click on Properties:
In the Symbol Property Editor, choose a symbol Type:
If need be you can now browse to different folders for different symbol sets or pictures. Note that the default Picture fills that come with ArcGIS are stored in the C:\Program Files\ArcGIS\Styles\Pictures folder (and you may need to navigate to this folder)
For a Picture fill, you will want to change to Thumbnails:
Choose a Picture fill and click Open
Modify the Angle, Scale, Color, etc as desired...
OR: For a Marker fill...
Click on the Marker... to choose a marker:
Select a Marker symbol, modify the Color, Size and Angle as desired...
Change the Offset and/or Separation (between symbols) as desired:
View your new custom background...
OPTIONAL - Move the Latitude-Longitude lines below the land features:
This makes the Lat-Long lines appear as part of the background...