Replace hardcoded white backgrounds with CSS variables

[bonheur15]

Problem

Several elements in the CSS use hardcoded background: #fff instead of referencing the CSS custom property system.

Location: CSS in internal/web/views/layout.templ

Affected Lines

• .builder (line 60): background: #fff
• .builder .out (line 109): implicit white background
• .results (line 141): background: #fff
• .config-ref (line 151): background: #fff

Proposed Fix

Define a --surface variable and use it consistently:

:root {
    --surface: #fff;
}

@media (prefers-color-scheme: dark) {
    :root {
        --surface: #161b22;
    }
}

.builder {
    background: var(--surface);
}

.results {
    background: var(--surface);
}

.config-ref {
    background: var(--surface);
}

Why This Matters

When dark mode is added (see related issue), hardcoded #fff backgrounds will need to be updated individually. Using a variable now makes that change trivial and prevents inconsistencies.