If you want to tether, you have a couple of options. Go to cydia and buy MyWi (or pirate MyWi).
As far as theming...there are themes you can pick up free in cydia, or you can buy in cydia. They all pretty much require you to download winterboard from cydia. Theming manually has a significant learning curve, but it's worth it to learn. I've had the same theme forever, just change up the wallpapers.
A few programs that I consider "must haves" on freshly jailbroken devices
SBsettings (quick access to toggle settings)
Winterboard (required for most themes)
Springtomize - A fantastic customization program
BiteSMS - best texting client there is imo
iFile - Access phone's root file directory