The new Gutenberg editor for may not work well with all existing WordPress themes. You may decide that you want to deactivate the editor and revert to the old editor. To deactivate, use the Classic Editor plugin.
Or, you can use both the classic and the Gutenberg editors. Install the Classic Editor plugin and amend the setting: “Use the Gutenberg editor by default and include optional links back to the Classic editor” As a result, you can use and learn the new Gutenberg editor while reverting to the Classic editor at any time.
The reason your theme doesn’t fully support Gutenberg is most probably because Gutenberg didn’t exist when your theme was built.
Your developer should be able to add the missing Gutenberg styles to your theme so that the blocks are styled correctly and match your theme.
Contact Webtree if you require any assistance with upgrading your current theme to work correctly with the new Gutenberg Editor.