ensure we normalize the arbitrary modifiers

This applies the same rules as arbitrary values. The `_` can be used in
place of a space. If you _do_ want an underscore, you can escape it with
`\_` (`\\_` in JavaScript).

src/util/pluginUtils.js

@@ -115,11 +115,7 @@ export function parseColorFormat(value) {
 }
 
 function unwrapArbitraryModifier(modifier) {
-  modifier = modifier.slice(1, -1)
-  if (modifier.startsWith('--')) {
-    modifier = `var(${modifier})`
-  }
-  return modifier
+  return normalize(modifier.slice(1, -1))
 }
 
 export function asColor(modifier, options = {}, { tailwindConfig = {} } = {}) {

tests/arbitrary-values.test.js

@@ -637,4 +637,19 @@ crosscheck(({ stable, oxide }) => {
       `)
     })
   })
+
+  it('should support underscores in arbitrary modifiers', () => {
+    let config = {
+      content: [{ raw: html`<div class="text-lg/[calc(50px_*_2)]"></div>` }],
+    }
+
+    return run('@tailwind utilities', config).then((result) => {
+      return expect(result.css).toMatchFormattedCss(css`
+        .text-lg\/\[calc\(50px_\*_2\)\] {
+          font-size: 1.125rem;
+          line-height: calc(50px * 2);
+        }
+      `)
+    })
+  })
 })