99% of the ringtones, themes and wallpapers on Cydia are utter wank so avoid at all costs and probably why your getting errors. I have those hidden in Cydia as doing so increases the speed of Cydia some what.
You need to know what apps to go for on Cydia like biteSMS, SBSettings and Firewall IP are my top 3 jailbroken apps and cant live without lol. Others I recommend are Infinifolders ( removes 12 app limit within folders), NoSpot (removes the search spotlight when you flick to right on Springboard), iBlueNova (can Bluetooth to any phone using this), iBlacklist (Block numbers from being able to call or text you), FolderCloser (Auto closes folder after app launch), Remove background and also Remove Backround sbsettings toggle ( Lets you remove all apps running in the background at once instead of having to do them one by one in the bottom dock after you've double clicked the home button)
If your wanting proper themes for your iPhone then sites like
http://macthemes.net/forum/viewforum.php?id=24 or
http://modmyi.com/forums/new-skins-themes-launches/ is where you need to visit.