Is LXDE theme can be modify? Yes, LXDE can use GTK 2.0 Engine theme, but where to get LXDE theme?, there's no lxde-loox.org, but you can get it on gnome-look.org. There are many GTK 2.0 Theme available.
On LXDE gtk not applied to window manager, because LXDE using openbox. To change window manager theme you can find on openbox theme on box-look.org. To make it easy install the obconf first.
$sudo pacman -S obconf
Okay first find the themes that you like, open gnome-look.org, search for gtk2 themes.
Here's for examples:
For download link:
http://gnome-look.org/content/download.php?content=156709&id=1&tan=5051085
Then extract, and paste the folder into .themes, if you don't have .themes folder just create it.
Then open Customize looks and feel. To apply the new theme.
Ok that's about GTK Themes, then how about window manager, you must open obconf, or openbox configuration manager. To install the themes is same as above lxde theme, just copy folder to .theme, but if you have obt file just install it from openbox configuration manager. Like this:
Then now choose the openbox themes to apply.
Ok that's it now choose the themes on box-look, or gnome-look.org that suitable with your style... :D