As Gutenberg nears a merge with WordPress, Theme Authors are running out of time to ensure that their themes are compatible. The Gutenberg handbook has an excellent article on how to opt-in and add support for enhanced features.
Most themes will present the default blocks without any issues as the blocks themselves provide their own styles. If you use a theme that does not fully support Gutenberg such as the Wide or Full Block Alignment options, the Theme Support for Gutenberg plugin may be for you.
Created by Weweaver, Theme Support for Gutenberg claims to allow most WordPress themes to be compatible with Gutenberg. In addition to theme support, the plugin adds a Classic Editor button to the admin bar to easily switch between Gutenberg and the Classic Editor.
Since a default WordPress theme is used to show how this plugin is beneficial, I decided to try it for myself. I installed the plugin on a fresh install of WordPress 4.9.7, Gutenberg 3.3, and the latest version of Twenty Sixteen. I published a Gutenberg Demo post which uses many of the default blocks.
Twenty Sixteen Looks Better Without It
Here is what the Gallery block looks like in Twenty Sixteen with the plugin disabled. The content looks good and fits within the content column.
When the plugin is enabled, the images are so large, a horizontal scroll bar appears.
The plugin includes additional styling for default blocks. One block that doesn’t benefit from the enhanced styles is the Video block. With the plugin disabled, the video block appears normally.
With the plugin enabled, the video block overflows the content column and breaches into the left sidebar.
Although the plugin initially shipped with no options, version 0.2 includes new settings that provide better compatibility for some themes. I checked the box to disable Fitvids support which solved the video block issue. However, the other options had no effect on the oversized image blocks.
Twenty Sixteen and Twenty Seventeen do not work well with this plugin. There may be themes, particularly older ones that benefit, but more modern themes will likely be ok without it.