I think it depends on a lot of social factors. For one, does it even affect you or your position if you do not adhere to the style guide? After all, a lot of code has been written by others with this guide in place already. If it does not really affect you, do you (for whatever reason) want to champion the style guide?
Most of the times, I found that once a style guide was ignored by one or more teams without serious repercussions, then the guide may as well be removed. It simply is not sufficient to write a document that explains the style guide, one needs to enforce it as well.
Hence, a more fundamental issue is present here: Does your company's style guide still have supporters, and why did they not enforce it?
You can think of it the other way round: You wouldn't have even bothered with this question, if your boss told you that must write any and all code for all projects in correspondence with this style guide. Apparently though, you have doubts as to the applicability of the guide instead.
In summary, what can you do? Given the above, these are a few possible ways, but in the end it is up to you to choose:
Ignore the style guide completely, because everyone else does so as well, and matter-of-factly it is no longer relevant.
Silently apply the style guide to your modifications and if someone complains simply point to the guide. You're not doing anything wrong, but you're also not doing much more than the minimum.
Champion the style guide. Bring up the problem that this code has ignored the guide, and that this fact in itself, is a problem that should be addressed. Get your voice heard and be part of the group that actually enforces the style guide, because you think it is important.
Challenge the style guide. Almost as above, but you don't agree with the guide and want to get rid of it.