I asked this recently and several people responded that it was not known in the community right now. Someone said they were going to try to find where that parameter was but that is the last I heard. I love this feature, but I too want a different color.
In the meantime, now I have a lovely black and green theme that matches it
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Settings/system/battery settings/battery indicator style/top bar. This is the thin line you are looking for. As for the color, it can probably be changed, the same way the circle battery indicator changes color. I don't theme so I don't know what is involved, but the people who do should be able to change this.