By default, a blank Omnyfy Install will not have a theme applied to it. You will need to apply the Porto theme through another section of the admin:
Content > Design > Configuration.
Once you reach the new page, you will then need to choose "Smartwave Porto" as the Applied Theme.
NOTE: "Smartwave Porto RTL" is the Porto Demo but with the reading direction, from Right to Left. This is used in certain countries that read information from right to left (such as Middle Eastern countries).
Then, click on Save Configuration on the top right to apply the theme (you can come back here later to configure the rest of the design to your liking).
NOTE: You can develop your own custom child theme based off the Porto Demo, with the parent of this custom theme to be "Smartwave Porto" (or "Smartwave Porto RTL" if applicable), to tweak/personalise your design. You can then select this custom theme in the "Applied Theme" field.
You can continue to configure the remaining fields in the Porto - Settings Panel and the Porto - Design Panel or the Design Configuration section, however at this point it would be easier to include content (dummy content for vendors, products and other Marketplace Configurations) so you can see the changes in the fields and what it affects. You can alter the Blocks and CMS pages after the import if needed.