Developing a WordPress Theme

Hellow, Guyz please help me i’ve create a wordpress theme for personal use. with these pages
header.php
sidebar.php
single.php
page.php
comments.php
But my problems is that, when a install a new plugin from wordpress and drop it in weight(sidebar). it does not show in proper way. By mean it just shwo without own css file. as it should show like it show on other sites with full html+css.
sorry for, poor english!
thankx for help