Templates
TweetDeck is using mustaches as a template engine (or a close derivative of it)
Location
TweetDeck templates are located in following objects:
window.TD.mustacheswindow.TD_mustaches(is an exact copy of the first one)
These objects include all templates and you can access their contents with simply calling their keys:
TD_mustaches['account_settings_account_summary.mustache']
Overriding Templates
It's not possible to completely override templates like this:
TD_mustaches[...] = "new template"
Instead you have to do it like this:
TD_mustaches[...] = TD_mustaches[...].replace("</div>", "</div> new template")
Notice
Templates should be overwritten as soon as TD_mustaches is available, modifying templates while they are in use can led to unexpected results.