[TYPO3-english] Backend module style is not work

[Rayuth You]

Hi list

Using typo3 4.5, I want to put css style in my backend module.
I put one line below in backend registre module of ext_tables.php
$TBE_STYLES['stylesheet'] = t3lib_extMgm::extRelPath($_EXTKEY) . 
'Resources/Public/CSS/backend.css'

But it does not work. My backend module has no css style.
What I do wrong? or What I need to do next?

Thanks,
Yuth