How can I use slim templates with Craft?
To answer your question: I think you CAN, but it will not be easy.

Craft is built on top of the Yii framework. It lets you override things (settings, classes) in the config/app.php file that gets installed by default. So, you should be able to change the view renderer to anything you want:

&lt;?php

return [
    'components' =&gt; [
        'view' =&gt; [
            'class' =&gt; 'craft\web\View',
            'renderers' =&gt; [
                'slim' =&gt; [
                    'class' =&gt; 'path\to\your\slim\ViewRenderer',
                ],
            ],
        ],
    ],
];


Of course, this would require a Yii ViewRenderer based on Slim, which I don't think is  available (yet). You could build it yourself, based on the Twig ViewRenderer used by Craft.

The reason I would not do it, is that Craft has several Twig filters en functions that make your templating life easier. You'd have to create wrappers for those, to be able to use them. I don't think the added complexity of all this weighs up to the advantage of using Slim.