feat: Add color fading text of Volume column

[Nemo55aa]

Pull request format:

This pull request aims to close issues #284 by adding "Volume color 2" configuration into Appearance Config Dialog.

This pull request corresponds to Dn-Programming-Core-Management/Dn-help#20. Please merge Dn-help's PR before this one.

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Related Issue detail for reference

Feature request:

• Flag in appearance settings to fade volume column color based on the volume value

If user didn't like that color fading, user can disable as setting color of "volume column 2" same as "volume column". so i thought this wouldn't need this.

• Would add an additional color to the below selector for the volume column to fade to.

Implementation checklist:

• Color option to selector UI
• Checkbox to enable behavior
• Implement in theme export/import
• The behavior itself

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Changes:

• Add Volume column color fading feature (@eulyderg Feature: Appearance settings > Volume column color fade #284 @Nemo55aa feat: Add color fading text of Volume column #419)

Example

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[eulyderg]

I would personally prefer for this behavior not to be enabled by default. This is what I had in mind when I said that there should be a checkbox to enable it. I think it's important to me that older themes still look the same way that they used to.

Is there a reason you decided against this?

[Nemo55aa]

I chose this implementation simply because it requires less line to impliment; there’s no deeper intent behind it.

I think that can handle by setting the default scheme's constant of "volume column 2" to the same fixed value of "volume column".

Would it still be better to include a checkbox?
Fixed. (Summary: I was just lazy at first push, apologize.)

[Nemo55aa]

Hi !
Just a quick update I have implemented the checkbox to toggle the volume color fading behavior, so themes will now retain their original appearance by default.

Let me know if the current implementation looks good to you or if there's anything else you'd like me to adjust! Thanks!