Themes
Typically themes are called templates within dokuwiki
Bootstrap3 Template
The current theme we're using is based on the bootstrap3 template / plugin. We're currently using the bootswatch slate theme that's been modified a bit to tidy things up on dokuwiki.
To be fair this is a bit old and crusty since bootstrap is now on version 5. But it does seem to work well / look nice and writing a new one would mean editing a lot of php probably.
If we wanted to use a different theme other than slate then there's a few listed in the bootswatch for bootstrap 3 we could use. Although personally I think the darker ones look cooler. There are one or two customisations I've added in but those could be copied / pasted over.
The path to the customised css is wiki\lib\tpl\bootstrap3\assets\custom\bootstrap.min.css
with some content added to the bottom of that file. The plugin did have an option for a theme switcher, but it seems broken at the moment due to the newer dokuwiki moving to lazy loading it's JS in an async way I think. I suspect it could be added back in if someone wanted to mess around a bit coding php.
Index Menu
The index menu has options for different types of icon, currently we're using the whiteblack one
{{indexmenu>:|navbar js#wb.png}}
There's a full list of all of them in this directory wiki\lib\plugins\indexmenu\images
Forcing a cache refresh
To force the site to refresh it's cache when editing CSS. Edit then slightly change the wiki/conf/local.php
file (usually just a newline or remove a newline from the end.