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Assuming you do not have a backup/Time Machine archive of the Mac before it was messed up... Insert the Mac OS X installation disc and hold down the C key while the Mac starts up. This will boot the Mac from the installation disc. Undo what you did to change the Mac Login screen. You kept a copy of the original files didn't you?
Press space bar
To change the screen resolution on a mac:Open system preferences by:clicking the apple menu at the top of the screen, then clicking on system preferencesClick on displaysHere, you will find all of the screen resolution settings that your Mac can support.
control alt command 8
The Desktop image can be changed in from Desktop tab in the Desktop & ScreenSaver section of System Preferences. You can jump directly to this by right clicking on the Desktop and selecting Change Desktop Background.... from the menu that appears.
http://www.ofzenandcomputing.com/fix-mac-blue-screen-boot/
maybe because your on white and black. :P there will be a help section the mac website or were you change the settings there will be some help.
On my MAC I go to System Preferences and then into desktop. At that point I choose a new picture for my screen. A similar command exists under Windows but I forgot what it is. I do not have my manual with me.
To set up Mac OS X to connect to a server at Login: Connect to the server you want to use. Open System Preferences. Select your account from the Accounts section. Click the Login Items tab. Drag the server's icon from a Finder window into the list of Login items in the System Preferences window. Quit System Preferences.
pocket additon: you cant Computer:choose a skin you like click on it then press upload to minecarft it will take you to minecraft home login if you have not allready press change then go on to minecraft and your skin has been changed!
If you have Mac OS X 10.6 (Snow Leopard) then QuickTime can record the screen - select New Screen Recording from the File menu. Try Snapz or screenflow, both of them record screen for Mac. or check more at http://en.wikipedia.org/wiki/Screencast
Go System Preferences > Accounts and set a login password. Go System Preferences > Security and under "General," uncheck the "require password... after sleep or screen saver begins" option.