fix color scheme with the Rust theme

[pinage404]

Hello,

When using light-dark() in CSS, it works great with all theme but not with the Rust theme

Others themes have the variable --color-scheme set to light or dark but the Rust theme don't have this variable

Before	After

Using this file :

<style>
    main {
        background-color: light-dark(white, black);
        color: light-dark(black, white);
    }
</style>

# title

Some content

I manually checked many pages, i don't see regression, i don't see changes if i don't use ligth-dark()

[GuillaumeGomez]

Changes look good to me. Please add a GUI test for it

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[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[GuillaumeGomez]

I added the missing GUI test.

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[pinage404]

Thanks =D