Keep2theCode

IMPORTANT: If you are updating this plugin from an older version, be sure to have backups of any stylesheets or icons, because you will need to re-upload them via the new form.
This plugin allows you to “tweak” or do minor overrides to the style for any theme, much like a child theme. But it also makes it easy to show images for your site’s logo, url, smartphone bookmark, and social sharing at sites such as Facebook.
For styles, first create a style sheet for the theme you want to adjust. For example, if you have a theme called soverycool, you might name your style sheet soverycool.css. Your file only needs to contain those style elements you’d like to change.
Next, upload it via the Upload File form. Once your file is uploaded, just insert its name into the css filename box and press Set Style. If you have several themes, you can keep all of your custom style sheets in that directory as long as they have different names. Then just enter the name for whichever theme you’re using at the time.
To stop using your custom style, just remove the name from the css filename box and press Set Style. You do not need to actually delete your files from the uploads directory.
For icons, upload them via the Upload File form and repeat for each icon. The names have to be favicon.ico, phone.png, share.png, and logo.png. Changing them is as easy as uploading new files with the same name. For your site logo to appear, the following code needs to be added to your theme’s header in the place you want it to appear: <code><?php thtw_logo(); ?></code>

Theme Tweak

Do minor changes to any theme styling, and easily display site images: logo, favicon, smartphone icon, and sharing icon.

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.

Theme Tweak vs TJ Custom CSS