chore: opt out of auto dark theme

Browsers can automatically generate dark themes from light themes, but it often
looks strange and Hubble is no exception. It's possible to opt out of automatic
dark themes by explicitly declaring the theme as light.

https://developer.chrome.com/blog/auto-dark-theme/#using-a-meta-tag

Fixes: cilium#664

src/index.html

@@ -4,6 +4,7 @@
     <meta charset="utf-8" />
     <title>Hubble UI</title>
     <base href="/" />
+    <meta name="color-scheme" content="only light">
     <meta http-equiv="X-UA-Compatible" content="IE=edge" />
     <meta
       name="viewport"