Good questions

1. It should work with any color schema. I created both Adaptive and Default theme, and it uses schema colors. It should be OK
2. Yes, this is how Sublime works, it is merging all sources together. I created file Packages/User/Adaptive.sublime-theme and all I inserted is code below. After saving, Sublime immediately applied those changes you requested above.

{
    "rules": [
        {
            "class": "sidebar_label",
            "parents": [{"class": "tree_row", "attributes": ["expandable", "expanded"]}],
            "color": "color(var(--white))"
        }
    ]
}

Whoa, this is great! I like it a lot. Thanks for the clarification, too. At first, I thought your package had the exception for expanded folders by default.

@arxeiss There's a small bug that I cannot work out. Expanding a .gitignored folder now un-dims it. I tried making an exception with

    {
        "class": "sidebar_label",
        "parents": [{"class": "tree_row", "attributes": ["expandable", "expanded", "!ignored"]}],
        "color": "color(var(--white))"
    }

but that isn't working. Do you know what I'm doing wrong?

@michaelblyons here:

{
    "rules": [
        {
            "class": "sidebar_label",
            "parents": [
            	{"class": "tree_row", "attributes": ["expandable", "expanded"]},
            	{"class": "file_system_entry", "attributes": ["!ignored"]}
            ],
            "color": "color(var(--white))"
        }
    ]
}

How can I use in other theme？

The rules are the same regardless of the theme you're using; you just need to change the name of the file that you create to make it apply to a different theme.

If you're using ST4. pick UI: Customize Theme and the file on the right is where you would put the rules. If you're using ST3 you can install PackageDev and use it's PackageDev: Edit Current Theme command. Alternately, the name of the file you want to create in your User package can be found by using Preferences > Settings and using the name of the file mentioned theme setting.

great works!