In my game, editing StarboxDefs.xml and/or editing GFX show up in game. Are you using the one in ...DLC/EXP2_Crusade/Game ? It needs to go in ...My Games/GC3Crusade/Mods 

Assuming you get it to show up, an alternative to deleting it is to use one with more contrast. Go to line 82-84 in StarboxDefs.xml, <RandomNebulaTexture> (that's the textures for the default starbox). Change all 3 to "invasion03.dds" It's a blue nebula that is much more readable. You can use any texture in the GFX folder. 

I'd guess if you just delete the texture path completely, it will default to stars (if the game doesn't crash). Not tested this:

<RandomNebulaTexture></RandomNebulaTexture>

The opacity numbers in line 79, 80 can also be adjusted for more clarity. I use:

    <MinNebulaOpacity>0.2</MinNebulaOpacity>

    <MaxNebulaOpacity>0.7</MaxNebulaOpacity>

Thanks. This gives me some hope. It seems like my edits of the StarboxDefs.xml file didn't take either because I was editing the wrong file or I put a copy of the edited file in the wrong place. I know Where do I put the .xml file? I tried putting a copy here but it didn't work when I edited it:  \Documents\My games\GC3Crusade\Mods. Thanks for the help.

I've reinstalled the game and verified integrity of the files. Then copied StarboxDef.xml file from steam's Data/Game folder and pasted it here:  \Documents\My games\GC3Crusade\Mods\NoNebula\Game. In the DefaultStarbox section I made the highlighted changes:  

 <NumRandomNebula>1</NumRandomNebula>

    <NebulaMaxMinRatio>1.0</NebulaMaxMinRatio>

    <NebulaFillRatio>8.0</NebulaFillRatio>

    <MinNebulaOpacity>.10</MinNebulaOpacity>

    <MaxNebulaOpacity>.10</MaxNebulaOpacity>

    <RandomNebulaTexture>Temp/Gfx/Space/invasion03.dds</RandomNebulaTexture>

    <RandomNebulaTexture>Temp/Gfx/Space/invasion03.dds</RandomNebulaTexture>

    <RandomNebulaTexture>Temp/Gfx/Space/invasion03.dds</RandomNebulaTexture>

Double checked that mods are enabled. Then tried starting a new game, quick start and loading a saved game. Still no changes to the background nebula. I don't know what I could be missing.

Changed <NumRandorNebula> from 1 to 0. Still does nothing. Nothing I do the the .XML file causes any changes. Something is stopping the game from using the file with the changes. Any ideas for next steps?

Are you starting a new game? xml changes do not update in games started previously, as the xml used during game creation is stored with the save game in Stardock games.

Any chance those changes are not over-riding the default game files? Perhaps try changing the default game files just to test. Verifying files will restore them to how they should be if you mess anything up.

If Mods are enabled, then I'd have to suggest that the mod isn't selected and/or the file path is wrong. 

you don't have to select it, if it is in the crusade mod folder and your playing crusade all mods in there should activate. If your not playing w/ crusade then it should be in \Documents\my games\GalCiv3\Mods

Outside of using the mod folder if your trying to change game files, you actually have to recompile (or something similar) after changing the file, as all that stuff is pre-compiled to help speed load times. My terminology may be incorrect but either here or on steam stardock posted the specifics. I know there was also a mod that helped remove the nebulae to but I haven't tested or tried it in ages.

Do not need a Data folder, /Mods/My Mod/Game is sufficent. (/My Mod/ is actually the "Data" folder)

I once saved my modded file as a .txt.  Everything looked fine in the editor.  Refused to work.  Another time I was caught up in the differences between Crusade and base game.  Started with a copy of the Crusade version and redid things from there.  Worked fine from there.

Actually, my nebula mod file started as a copy of the original, just as you describe.  Didn't have the gumption to start from scratch!  Guessed at what I was doing til it worked!