Theme Junkie

This plugin will enable a <a href="https://www.theme-junkie.com/plugins/theme-junkie-custom-css/" rel="nofollow ugc">custom css manager</a> on administration page to add Custom CSS code to your WordPress website. It will automatically override any theme or plugin default styles. It also very useful if you want to add customization to your website but do not want to edit your theme or plugin css files.
It comes with two ways to add the custom css code:
1. Setting 
You can go to Appearance → Custom CSS, then you will see a big box/textarea. Put your css code there.
2. Customizer – Live Preview 
If you want to see the live preview while you adding the custom css code, then you can go to Appearance → Customize, after that open the Custom CSS section tab.
<h4>Features Include:</h4>
<ul>
<li>No configuration needed</li>
<li>Live preview</li>
<li>Easy-to-use</li>
<li>Child theme alternative to add customization</li>
<li>Uninstall procedure</li>
</ul>
<h4>Plugin Info</h4>
<ul>
<li>Developed by <a href="https://www.theme-junkie.com/?utm_source=wporg&utm_medium=text_link&utm_campaign=Site%20Promotion" rel="nofollow ugc">Theme Junkie</a></li>
<li>Check out the <a href="https://github.com/themejunkie/theme-junkie-custom-css" rel="nofollow ugc">Github</a> repo to contribute.</li>
</ul>

TJ Custom CSS

Simply works

Easily to add any Custom CSS code to your WordPress website.

Themeisle

Hardeep Asrani

Blocks CSS allows you add custom CSS to your Blocks straight from the Block Editor (Gutenberg).
It adds a syntax-highlighted CSS Editor where you can add additional CSS to your Gutenberg Blocks to style them the way you want.
All the code and sources for this plugin are publicly available as part of https://github.com/Codeinwp/otter-blocks.

Blocks CSS: CSS Editor for Gutenberg Blocks

This is must have plugin for Gutenberg Block Editor. Makes it so much easier to add css classes where you use the blocks! 

Excellent!

I think I just would not be able to use the block editor without this. Thank you so much for this plugin!

This plugin saved my life!


This is a great tool for what it does. The only confusing thing for me is that it says on .org that you have 11 blocks.  I couldn't spot any new ones after installing this plugin, nor found any settings.


This is a great tool for those wanting to use core blocks


Can't think of Block Editor without this. Tiny powerful thing. 



Only thing I hate in this plugin is the following line:



$render_css = empty( $file_name ) || strpos( $file_name, 'post-v2' ) === false;



This means if you deactivate Otter, your content might just break :( 


Completes Block Editor

Simple and great plugin!

Blocks CSS allows you add custom CSS to your Blocks straight from the Block Editor (Gutenberg).

TJ Custom CSS vs Blocks CSS: CSS Editor for Gutenberg Blocks