Theme update

Hi, Another question. We are hosting the website www.worldcupbrazil.net on Bluehost. Occasionally the website encounters Error 520, 522 or 524. Then the website is offline for a few seconds. We contacted Bluehost in order for them to solve this issue. They checked on http://www.worldcupbrazil.net/error_log and stated it is the theme which causes these errors. A theme update may solve the issue the support desk stated. We want to install the theme update (2.9.1) and the Wordpress update (3.8). But the thing is, we're worried all the css code and other modified code will be overwritten? We don't use any child theme so...is it still possible to install these updates and keep our current code? Thanks in advance Joao