Allow custom override of code env styling

[Delivers #64114122]
Load styles in right order

[Delivers #64114122]
Recover from MessagePack::UnpackError when reading from highlight cache

[Delivers #64492504]
[#64492504]

* 64492504-remove-highlight:
  Recover from MessagePack::UnpackError when reading from highlight cache
  Load styles in right order
  Allow custom override of code env styling