chore: upstream some theme elements and update Verso

This should fix horizontal scrolling on mobile phones.

lake-manifest.json

@@ -25,10 +25,10 @@
    "type": "git",
    "subDir": null,
    "scope": "",
-   "rev": "6b15732648542ff28017eea75da3dddceea1fe91",
+   "rev": "fb7cd904d704155f793c9c5798e4ba08b1efe739",
    "name": "verso",
    "manifestFile": "lake-manifest.json",
-   "inputRev": "main",
+   "inputRev": "mobile-css-manual",
    "inherited": false,
    "configFile": "lakefile.lean"}],
  "name": "«verso-manual»",

lakefile.lean

@@ -7,7 +7,7 @@ import Lake
 open Lake DSL
 open System (FilePath)
 
-require verso from git "https://github.com/leanprover/verso"@"main"
+require verso from git "https://github.com/leanprover/verso"@"mobile-css-manual"
 
 package "verso-manual" where
   -- work around clang emitting invalid linker optimization hints that lld rejects

static/theme.css

@@ -17,42 +17,6 @@ header h1 {
     margin: 10px;
 }
 
-h1 {
-  font-size: 3.0em;
-  line-height: 1.5;
-  margin-bottom: 1.5rem;
-}
-
-h2 {
-  font-size: 1.8rem;
-  line-height: 1.5;
-  margin-bottom: 1rem;
-}
-
-h3 {
-  font-size: 1.4rem;
-  line-height: 1.5;
-  margin-bottom: 0.2rem;
-}
-
-h4 {
-  font-size: 1.2rem;
-  line-height: 1.25;
-  margin-bottom: 0.2rem;
-}
-
-h5 {
-  font-size: 1rem;
-  line-height: 1.25;
-  margin-bottom: 0.2rem;
-}
-
-h6 {
-  font-size: 1rem;
-  line-height: 1.15;
-  margin-bottom: 0rem;
-}
-
 .hl.lean.block {
     margin-top: 1em;
     margin-bottom: 1em;