Feat: update line height

Since we have different font sizes, we should make the line height
dependent on the font size and not a fixed value. The recommended
value for accessibility is 1.5.
https://www.w3.org/WAI/WCAG21/Understanding/text-spacing.html

Signed-off-by: Marco Ambrosini <marcoambrosini@proton.me>

apps/theming/css/default.css

@@ -50,7 +50,7 @@
   --font-face: system-ui, -apple-system, 'Segoe UI', Roboto, Oxygen-Sans, Cantarell, Ubuntu, 'Helvetica Neue', 'Noto Sans', 'Liberation Sans', Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol', 'Noto Color Emoji';
   --default-font-size: 15px;
   /* 1.5 x font-size for accessibility */
-  --default-line-height: 24px;
+  --default-line-height: 1.5;
   --animation-quick: 100ms;
   --animation-slow: 300ms;
   /** Border width for input elements such as text fields and selects */

apps/theming/lib/Themes/DefaultTheme.php

@@ -161,7 +161,7 @@ class DefaultTheme implements ITheme {
 			'--font-face' => "system-ui, -apple-system, 'Segoe UI', Roboto, Oxygen-Sans, Cantarell, Ubuntu, 'Helvetica Neue', 'Noto Sans', 'Liberation Sans', Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol', 'Noto Color Emoji'",
 			'--default-font-size' => '15px',
 			// 1.5 * font-size for accessibility
-			'--default-line-height' => '24px',
+			'--default-line-height' => '1.5',
 
 			// TODO: support "(prefers-reduced-motion)"
 			'--animation-quick' => '100ms',