fix: move baseline css from mui to index.css (#22238)

This pull-request moves our baseline CSS styles from the MUI theme
(`site/src/theme/mui.ts`) definition to `index.css`. As these are global
styles they should live in one dedicated place not two.

site/src/index.css

@@ -108,6 +108,33 @@
 		@apply border-border;
 	}
 
+	html,
+	body,
+	#root,
+	#storybook-root {
+		@apply h-full;
+	}
+
+	button,
+	input {
+		@apply font-sans;
+	}
+
+	input:-webkit-autofill,
+	input:-webkit-autofill:hover,
+	input:-webkit-autofill:focus,
+	input:-webkit-autofill:active {
+		-webkit-box-shadow: 0 0 0 100px hsl(var(--surface-primary)) inset !important;
+	}
+
+	::placeholder {
+		@apply text-content-secondary;
+	}
+
+	fieldset {
+		@apply border-none p-0 m-0 w-full;
+	}
+
 	/*
 	By default, Radix adds a margin to the `body` element when a dropdown is displayed,
 	causing some shifting when the dropdown has a full-width size, as is the case with the mobile menu.

site/src/theme/mui.ts

@@ -5,46 +5,13 @@
 
 /** @deprecated MUI ThemeOptions is deprecated. Migrate to Tailwind CSS theme system. */
 import type { ThemeOptions } from "@mui/material/styles";
-import {
-	BODY_FONT_FAMILY,
-	BUTTON_LG_HEIGHT,
-	BUTTON_MD_HEIGHT,
-} from "./constants";
+import { BUTTON_LG_HEIGHT, BUTTON_MD_HEIGHT } from "./constants";
 import tw from "./tailwindColors";
 
 // biome-ignore lint/suspicious/noExplicitAny: needed for MUI overrides
 type MuiStyle = any;
 
 export const components = {
-	MuiCssBaseline: {
-		styleOverrides: (theme) => `
-      html, body, #root, #storybook-root {
-        height: 100%;
-      }
-
-      button, input {
-        font-family: ${BODY_FONT_FAMILY};
-      }
-
-      input:-webkit-autofill,
-      input:-webkit-autofill:hover,
-      input:-webkit-autofill:focus,
-      input:-webkit-autofill:active  {
-        -webkit-box-shadow: 0 0 0 100px ${theme.palette.background.default} inset !important;
-      }
-
-      ::placeholder {
-        color: ${theme.palette.text.disabled};
-      }
-
-      fieldset {
-        border: unset;
-        padding: 0;
-        margin: 0;
-        width: 100%;
-      }
-    `,
-	},
 	MuiAvatar: {
 		styleOverrides: {
 			root: {