Browser features not matching our dark mode

[GCHQ-Developer-741]

Summary of the bug

Some browser specific features (such as scroll bars on certain elements) still have a light scroll bar when the system settings are light, even when a dark theme is being applied through ic-theme.

📸 Screenshots or code

🧐 Expected behaviour

When setting our dark mode, browser features should appear in dark mode too.

Additional info

Can potentially be solved by adding a color-scheme property to our .ic-theme-dark class (https://www.amitmerchant.com/darkmode-aware-scrollbars-using-css/)