Adapted by from a post
by Sarah Coleman
For the purpose of this post, let's assume that your existing template is subSilver and that you want to create a template called yourname.
(Replace yourname with the name of your template - one word).
Start by making a copy of the directory called subSilver, which is located inside the "templates" directory of phpBB 2. Call the new directory "yourname".
Inside "yourtemplate," there will be two files that you need to rename:
subSilver.cfg - rename to yourname.cfg
subSilver.css - rename to yourname.css
DO NOT rename theme_info.cfg.
After that, you need to replace all instances of the words subSilver with the word yourname, inside the "yourname" directory. Use your HTML editor or other software to do a "search and replace" because there are at least a couple of hundred!
Now you have to change the images. Open each one in your graphics editor and make whatever changes you want to - don't forget to make sure you're editing the images inside the yourname directory - NOT the subSilver directory!
When you're done with the images, upload the entire "yourname" directory to the templates directory on your server and chmod yourname/theme_info.cfg to 666.
Go into your Administration Panel on phpBB 2 and click on "Create" under Styles Admin. Select yourname from the drop-down list and complete the form using the values that you want - don't worry - you can tweak and make changes later on if it doesn't look the way you want it to at first.
When you've done that, click "Save Settings" at the bottom. phpBB 2 will tell you that you now need to export the theme.
To do that, click 'Export' under Styles Admin and select yourname from the drop-down menu. phpBB 2 should be able to install the theme automatically and report back to you that the export was successful.
Now you should be able to go into the "Configuration" panel under General Admin and select yourname from the drop-down list of available templates.
It's important to remember before you start that this isn't a quick job. It's going to take a fair bit of time and patience and trial and error to get your board looking just right - but it's worth it!
If after you've seen your new theme you decide that you need to make adjustments, you'll have to chmod theme_info.cfg back to 666 again before you edit it in the Administration Panel. Also, the Admin Panel won't change everything you want to change, you will most likely have to make some changes to yourname.css.
When you've got everything the way you want it, you should ALWAYS chmod theme_info.cfg back to 644 to prevent accidents or malicious attacks on your template.