It would be great if any theme you activated looked exactly like the demo once it was activated?
So if you’ve installed a theme (and it’s sample data) but your site isn’t matching up to the theme demo there are a couple very common reasons why your homepage might look different.
Homepage Display Error Solution 1
Reading settings are wrong. This means your theme either does or doesn’t require a home page to be set, and you have your site set to the opposite. So go to Settings > Reading and check the options under “Front Page Displays.”
If your theme doesn’t require any page template for the homepage layout make sure this option is set to”Your latest posts.” If your theme does use a homepage template, have your site set to use a static homepage.
Homepage Display Error Solution 2
You need to use a homepage template. A lot of theme developers use custom page templates for their homepage layouts. So you should first of course read the documentation files, but if there aren’t any head over and create a new page, while in the editor under Page Attributes > Template check to see if there is a Home, Homepage, Frontpage or similar template. If so, then you likely need to create a page using this template, save/publish it, then head over to Settings > Reading to set it as your static front page option.