For some reason Semantic-ui's grid is 16 wide and I'm working on an Orchard Cms theme for that (couldn't find one elsewhere).
In case anyone else is looking. I found that currently as of Orchard CMS 1.10.2 you can do this on the front-end but not the backend.
Here is an example of how the front-end grid was changed for bootstrap: https://github.com/OrchardCMS/TheBootstrapMachine/tree/master/Views/Elements
And the reason you can't change the admin page grid: https://github.com/OrchardCMS/Orchard/blob/6720b71cf3474a9a7b8a8cc9a99d58b1e733acfa/src/Orchard.Web/Modules/Orchard.Layouts/Elements/Grid.cs#L6
As you can see 12 is hardset in the code. I suppose you could extend the entire layout module, but this would be a lot of work because several other modules depend on it.
I plan to use Semantic-ui anyway but using their grid using this nomenclature:
<div class="ui twelve column grid">
<div class="four wide column">four</div>
<div class="four wide column">four</div>
<div class="four wide column">four</div>
</div>