How to get rid of ‘close or esc key’ in thickbox?

This is connected with wp-simpleviewer plugin, but the thickbox.js file comes bundled with WordPress so I am posting here.

I would like to change the ‘close or esc key’ to an X or a simple ‘close’.

I am trying to find the phrase in thickbox.js and .css but it’s not there. Somewhere I’ve read that one can download the thickbox.js file from its author and change those words. They are in fact there but when I replace the wordpress-bundled .js file with the ‘original’ thickbox.js the plugin doesn’t work.

What is strange is that the bundles .js file doesn’t have the ‘close or esc key’ phrase. So how come it is displayed? Should I look at another file? A css one?

Well anyway, I would like to modify those words. Help please