riverlea - fix checkbox-list in hackneybrook dark mode

[ufundo]

Overview

Fix issue with checkbox-list colours in Hackney Brook dark mode. I think this got garbled somewhere along the lines of versions of #32209

Before

After

Technical details

The existing defintion of checkbox-list-col isn't doing anything, it's being overridden two lines below

[civibot]

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[ufundo]

Have also backported to 6.0 branch here #32388

CC @vingle @colemanw