Looks like your current theme messed up the Gutenberg block style CSS. As u/devilmaydance mentioned, consider changing your theme. Perform a quick check by changing your current theme to WordPress default theme to see whether the problem is solved or not. If it solved your problem, then you can surely say that the issue is with the theme. I would avoid using badly coded themes for security reasons.
Try Generatepress with Elementor plugin; both have free versions to use. If you find Elementor is bulky for your current hosting plan, I would suggest using “Ultimate Addons for Gutenberg” (link to the plugin – [https://wordpress.org/plugins/ultimate-addons-for-gutenberg/](https://wordpress.org/plugins/ultimate-addons-for-gutenberg/)).
In case you want to add any custom CSS to your blog, go to Appearance->Customize->Additional CSS. Adding CSS this way preserves your code even when the theme is updated.
Also, try clearing your browser cache or reload the page using “Ctrl/Cmd + F5” to see if it is a cache issue.