How does it work
SchemingMind Tuning internal details.
General idea
The extension works by modifying schemingmind pages. It is called by the browser whenever some page is loaded. If this is a schemingmind page, it modifies it in place, according to the preferences - substitutes original images with the configured ones, resizes some elements, paints toolbar above the title etc. This process is usually invisible, but in some rare cases (slow connection, main site under load) you may notice it.
The modifications are made in a simple manner - elements to modify are located using either internal names, or pieces of text, then patched as necessary. For example, to substitute the images, the extension locates all img elements on the page which has source address containing images/sets, then replaces the image address with the address of replacement set.
It is worth mentioning that the extension does not understand/interpret games/moves.
Technical details
If you are interested, you may read the source code. Download the XPI file (just right-click the Download link and pick Save As), unpack it (it is a normal ZIP file, rename it from smtuning.xpi to smtuning.zip and unpack), then unpack the chrome/smtuning.jar file (again, this is a zip file). All the important sources are in the content directory, it is a mix of XUL (mozilla interface language, in which I model the options dialog and also declare scripts used) and JavaScript (in which all the functionality is implemented). Main code is in overlay.js file, start reading from the onPageLoad routine - this is a routine called whenever browser finishes loading some page.