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I am not sure if you can hex edit it, but that is part of the steam 971.png image which has a lot of transparency in it I have overlaid the steam image over a gray background to show you where it's at.
If you are using the default aero theme I can change that for you. I use Windows Style Builder Ave's Windows7 Style Builder
to edit themes which separates that steam image making it easier to edit.
If you are using a custom theme you will need to ask the
theme maker. It's not cool to change someone else work.
You will need to do 1 of these 2 ways of Enabling custom themes for it to work. Enabling Custom Themes - Windows 7
Or Theme - How to Apply Custom Themes