Fix Custom CSS Disappears After Saving

[KazumiHARADA]

Description

• Removed the default value of the customCSS on the ReactCodeMirror TAG in order to enable stored the CustomCSS value.
• Moved the default value to the ConfigManger.js.

Issue fixed

#3041

Type of changes

• 🔘 Bug fix (Change that fixed an issue)
• ⚪ Breaking change (Change that can cause existing functionality to change)
• ⚪ Improvement (Change that improves the code. Maybe performance or development improvement)
• ⚪ Feature (Change that adds new functionality)
• ⚪ Documentation change (Change that modifies documentation. Maybe typo fixes)

Checklist:

• 🔘 My code follows the project code style
• ⚪ I have written test for my code and it has been tested
• 🔘 All existing tests have been passed
• 🔘 I have attached a screenshot/video to visualize my change if possible

Screenshot

Tested

MacOS Mojave (10.14.5)

[ZeroX-DG]

The /* Drop Your Custom CSS Code Here */ was there to fix this issue: #2789

With your current code, if the user clears the custom CSS editor, the old issue will appear again. Can you make the /* Drop Your Custom CSS Code Here */ into a placeholder or something?

[KazumiHARADA]

I've modified inserting the default value to the custom CSS field when the field was saved empty.
Please check my code again.

[ZeroX-DG]

LGTM 🎉