I've modified the default "Mac Classic" theme and added "bold" to all "fontStyle" settings: Mac Classic Bold.tmTheme.zip
Have a look at the manual to see how you can install this file: http://www.textasticapp.com/v4/manual/lessons/How_...
You can modify the other themes in the same way. The default TextMate themes are available from the TextMate SVN repository: http://svn.textmate.org/trunk/Themes/ - others can be found on the internet.
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