how to install themes and mods?

Themes (called Skins in IPB world):

Go To: AdminCP > Look & Feel > Import/Export
On the 'import' tab you will see 'Upload XML(Archive) Image Set', browse to your .XML file for the theme (skin) and import it first. After that, import the skin set for the same theme (skin) using the box area under that. Once you have both of those, go to AdminCP > Look & Feel > Manage Skin Sets & Templates, you should see your newly installed theme (skin) in the list. Go to the far right dropdown box and select 'Edit Setting', scroll down the page till you see 'Use image directory?' and make sure you have the correct theme (skin) image directory selected for your installed theme (skin).

Mods:
It depends on the modification type, there are 3 types really.

1. Applications:
Upload the contents of the application directory for directory from your .zip/.rar package. After you have uploaded the files...
Go To: AdminCP > System > Applications & Modules > Manage Applications & Modules
You should see the application in the right hand box, click on the 'Install' button to install it. Don't forget that some applications require additional file edits to complete the installation.

2. Hooks:
Go To: AdminCP > System > Applications & Modules > Manage Hooks
The first tab on the list (not the default tab, you have to click it) is 'Install a New Hook'. Click on that, browse to find your hookname.xml file and install it.

3. Full Modifications:
These are more difficult as they require you to do file edits, just be sure to download a backup copy of your files before doing the edits. Follow each step paying special attention to 'Add Above' and 'Add Below' instructions.

Good luck