Add theme preview (#20039)

This PR adds a theme preview tab to help get an at a glance overview of
the styles in a theme.

![CleanShot 2024-10-31 at 11 27
18@2x](https://github.com/user-attachments/assets/798e97cf-9f80-4994-b2fd-ac1dcd58e4d9)

You can open it using `debug: open theme preview`.

The next major theme preview PR will move this into it's own crate, as
it will grow substantially as we add content.

Next for theme preview:

- Update layout to two columns, with controls on the right for selecting
theme, layer/elevation-index, etc.
- Cover more UI elements in preview
- Display theme colors in a more helpful way
- Add syntax & markdown previews


Release Notes:

- Added a way to preview the current theme's styles with the `debug:
open theme preview` command.

crates/ui/src/utils/color_contrast.rs

@@ -0,0 +1,70 @@
+use gpui::{Hsla, Rgba};
+
+/// Calculates the contrast ratio between two colors according to WCAG 2.0 standards.
+///
+/// The formula used is:
+/// (L1 + 0.05) / (L2 + 0.05), where L1 is the lighter of the two luminances and L2 is the darker.
+///
+/// Returns a float representing the contrast ratio. A higher value indicates more contrast.
+/// The range of the returned value is 1 to 21 (commonly written as 1:1 to 21:1).
+pub fn calculate_contrast_ratio(fg: Hsla, bg: Hsla) -> f32 {
+    let l1 = relative_luminance(fg);
+    let l2 = relative_luminance(bg);
+
+    let (lighter, darker) = if l1 > l2 { (l1, l2) } else { (l2, l1) };
+
+    (lighter + 0.05) / (darker + 0.05)
+}
+
+/// Calculates the relative luminance of a color.
+///
+/// The relative luminance is the relative brightness of any point in a colorspace,
+/// normalized to 0 for darkest black and 1 for lightest white.
+fn relative_luminance(color: Hsla) -> f32 {
+    let rgba: Rgba = color.into();
+    let r = linearize(rgba.r);
+    let g = linearize(rgba.g);
+    let b = linearize(rgba.b);
+
+    0.2126 * r + 0.7152 * g + 0.0722 * b
+}
+
+/// Linearizes an RGB component.
+fn linearize(component: f32) -> f32 {
+    if component <= 0.03928 {
+        component / 12.92
+    } else {
+        ((component + 0.055) / 1.055).powf(2.4)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use gpui::hsla;
+
+    use super::*;
+
+    // Test the contrast ratio formula with some common color combinations to
+    // prevent regressions in either the color conversions or the formula itself.
+    #[test]
+    fn test_contrast_ratio_formula() {
+        // White on Black (should be close to 21:1)
+        let white = hsla(0.0, 0.0, 1.0, 1.0);
+        let black = hsla(0.0, 0.0, 0.0, 1.0);
+        assert!((calculate_contrast_ratio(white, black) - 21.0).abs() < 0.1);
+
+        // Black on White (should be close to 21:1)
+        assert!((calculate_contrast_ratio(black, white) - 21.0).abs() < 0.1);
+
+        // Mid-gray on Black (should be close to 5.32:1)
+        let mid_gray = hsla(0.0, 0.0, 0.5, 1.0);
+        assert!((calculate_contrast_ratio(mid_gray, black) - 5.32).abs() < 0.1);
+
+        // White on Mid-gray (should be close to 3.95:1)
+        assert!((calculate_contrast_ratio(white, mid_gray) - 3.95).abs() < 0.1);
+
+        // Same color (should be 1:1)
+        let red = hsla(0.0, 1.0, 0.5, 1.0);
+        assert!((calculate_contrast_ratio(red, red) - 1.0).abs() < 0.01);
+    }
+}