highlight_themes_css causes Crash when specifying a theme file that is not bundled with zola.

[Jieiku]

Bug Report

This crashes because this syntax theme is not bundled with zola.

highlight_theme = "css"
highlight_themes_css = [
  { theme = "abridge-dark", filename = "syntax-abridge-dark.css" },
  { theme = "abridge-light", filename = "syntax-abridge-light.css" },
]

Environment

Zola version: 0.15.3

Expected Behavior

I did not see any note in the documentation that you can only use this feature with bundled themes.

Current Behavior

export RUST_BACKTRACE=full

zola build                                                                                                                                                         ✔ 
Building site...
Checking all internal links with anchors.
> Successfully checked 1 internal link(s) with anchors.
-> Creating 11 pages (0 orphan) and 2 sections
thread 'main' panicked at 'no entry found for key', components/config/src/config/markup.rs:78:33
stack backtrace:
   0:     0x564fa8b3beca - <unknown>
   1:     0x564fa8638b9c - <unknown>
   2:     0x564fa8b3a814 - <unknown>
   3:     0x564fa8b3af80 - <unknown>
   4:     0x564fa8b3a2ea - <unknown>
   5:     0x564fa8b6cd58 - <unknown>
   6:     0x564fa8b6ccd6 - <unknown>
   7:     0x564fa8b6cc92 - <unknown>
   8:     0x564fa8417be0 - <unknown>
   9:     0x564fa863d040 - <unknown>
  10:     0x564fa8417e3b - <unknown>
  11:     0x564fa85deee3 - <unknown>
  12:     0x564fa8b02972 - <unknown>
  13:     0x564fa85419bf - <unknown>
  14:     0x564fa84d97d3 - <unknown>
  15:     0x564fa8544219 - <unknown>
  16:     0x7f197252f290 - <unknown>
  17:     0x7f197252f34a - __libc_start_main
  18:     0x564fa849995e - <unknown>
  19:                0x0 - <unknown>

Step to reproduce

This will reproduce it:

highlight_theme = "css"
highlight_themes_css = [
  { theme = "abridge-dark", filename = "syntax-abridge-dark.css" },
  { theme = "abridge-light", filename = "syntax-abridge-light.css" },
]

Other Notes

I am motivated to use highlight_theme = "css" because I have enabled a Content Security Policy Header, and it is considered undesirable by Mozilla observatory to have style-src 'unsafe-inline':

https://observatory.mozilla.org/analyze/abridge.netlify.app

[Jieiku]

It definitely seems to be an issue only with custom themes, the built in one seem to work fine.

I compiled zola with my themes included, and was able to generate the css files no problem.

I included them here: https://github.com/Jieiku/abridge/tree/master/sass/syntax