I suspect that if you enable the Network tab in your browser Diagnostics screen (usually F9, but right-click and select ‘inspect’). you will see that a CSS file is not getting loaded for some reason.
This is probably a theme issue of not properly loading the CSS. Could also be a caching issue; perhaps the CSS file is cached and needs to be refreshed.
But looking at the items ‘requested’ via the Developer’s Network tab will probably show a ‘404’ on a particular CSS request. You can then look at that request and determine it’s “caller” (the code file that tried to load the CSS), and troubleshoot from there. As mentioned, I suspect it will be a CSS request made by the theme.
You could try a different theme; or clear your browser cache. Browser cache is likely if you are developing the theme and make changes to the CSS file. There are ways to add a parameter to the CSS request to ‘bust’ the cache, forcing the CSS file to be loaded from source rather than cache. You may also need to disable any caching plugin you have.