Inspired by the guy in this video, this morning I decided to flash the menus on my SE W810i phone and customize a unique theme. Steps are very simple. First you need to install a software called XS++ to be able to access the phone's firmware, then get .swf flash menu files and finally upload them in the phone using XS++ (in FXS mode) - the trick part is knowing in which folder you need to upload them (/tpa/preset/system/desktop/flash). Finally you need to modify the themes (which are basically POSIX tar files) to use the desired flash menu. Simple as that, really!