This is not a DokuWiki-specific question and all solutions you'll find on the internet will apply. There are several solutions, but the best on IMHO is the one I use in the starter template (and which is therefore also used in the dokuwiki template):
See the explanation in the comments of its main.php:
https://github.com/selfthinker/dokuwiki_template_starter/blob/master/main.php#L30
A slight variation of that is also used by the
html5 boilerplate.
It uses
Conditional Comments to set certain classes you can use in any CSS files to target specific IE versions.
The solution turnermm posted is one of the many solutions, but a really bad one. It uses browser sniffing, which is considered bad practice and might be punished by search engines. And in the DokuWiki-specific case it also means that you wouldn't be able to make use of our CSS dispatcher/preprocessor which would result in lost functionality and also more http requests.