Maximum font size slider bar didn't show when using dark mode

[barnett617]

Describe the issue

After switching to dark mode, the slider bar in Prefs tab is hard to see.
As the image below:

I find that something of _utilities.scss may cause this (just my guess) because the whole sidebar's background color seems strange.
I tried to disable the background color attribute setting, it seemed to be normal and the slider appeared as below.

Hope you find the real cause and fix it to support more good using in dark mode.

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