fix(ssr): remove color defaults from :root in _themes.scss

The SSR server injects fresh color CSS custom properties as an inline
<style> block before </head>, but _themes.scss also set hardcoded
color defaults on :root at build time. The two :root blocks compete
in the cascade — the compiled stylesheet (loaded after the inline
<style>) overwrites the server-injected colors, causing a visible
flash where wrong colors render briefly before ThemeService.apply()
corrects them.

Removing the color properties from :root in _themes.scss makes the
server-injected <style> the sole source of truth for colors. Non-color
properties (fonts, radii, shadows) remain as build-time defaults since
they don't change between deploys.

src/styles/_themes.scss

@@ -3,33 +3,15 @@
 // ========== CSS Custom Properties (exposed to the browser) ==========
 // These map SCSS variables to CSS custom properties so Angular components
 // can consume them via var() at runtime.
+//
+// COLOR variables (color_*, gradient_*) are NOT set here anymore.
+// They are injected at runtime by the SSR server as an inline <style> block
+// so the first paint always carries the correct station colors.
+// Client-side ThemeService.apply() keeps them in sync after Angular boots.
+//
+// Non-color variables (fonts, radii, shadows) remain below as build-time defaults.
 
 :root {
-  // Primary
-  --color-primary:          #{$primary-blue};
-  --color-primary-light:    #{$primary-blue-light};
-  --color-primary-dark:     #{$primary-blue-dark};
-  --color-primary-muted:    #{$primary-blue-muted};
-
-  // Accent
-  --color-accent:           #{$accent-orange};
-  --color-accent-light:     #{$accent-orange-light};
-  --color-accent-dark:      #{$accent-orange-dark};
-  --color-accent-muted:     #{$accent-orange-muted};
-
-  // Neutrals
-  --color-white:            #{$neutral-white};
-  --color-cream:            #{$neutral-cream};
-  --color-light:            #{$neutral-light};
-  --color-medium:           #{$neutral-medium};
-  --color-dark:             #{$neutral-dark};
-  --color-black:            #{$neutral-black};
-
-  // Semantic
-  --color-success:          #{$success-green};
-  --color-danger:           #{$danger-red};
-  --color-info:             #{$info-blue};
-
   // Typography
   --font-primary:   #{$font-primary};
   --font-heading:   #{$font-heading};