When installing the Loki Checkout for Luma, it could be that the styling is off.
First, make sure that the additional module for Luma is installed and enabled:
composer require loki-checkout/magento2-luma
bin/magento LokiCheckout_Luma
Next, make sure that your public static content is up-to-date. In the Developer Mode, this is easy - just rerun the set command will reset your environment:
bin/magento deploy:mode:set developer
For the Production Mode, we recommend you to follow proper deployment steps. In short, this involves rebuilding the static content and recompiling DI-based code.