According to <https://html.spec.whatwg.org/multipage/semantics.html#meta-theme-color>, we need to parse `content` as a CSS `<color>` value <https://html.spec.whatwg.org/multipage/infrastructure.html#parsed-as-a-css-color-value>, which explicitly mentions `currentcolor`.
<rdar://problem/74814749>
`<meta name="theme-color" content="...">` should also support CSS `hwb()`, `lch()`, and `oklch()` color functions (https://drafts.csswg.org/css-color/#typedef-color).