highlight_themes_css and extra_syntaxes_and_themes are incompatible if highlight_theme == "css"

[bahlo]

Bug Report

You can't use themes from extra_syntaxes_and_themes with highlight_themes_css. The extra_syntaxes_and_themes import works if I remove the highlight_themes_css statement and use highlight_theme = "base16-unikitty-dark" (for example).

Environment

Zola version: 0.15.3

Expected Behavior

Zola builds the CSS for the light and dark theme and writes it to static/.

Current Behavior

$ RUST_BACKTRACE=full zola build
Building site...
Checking all internal links with anchors.
> Successfully checked 0 internal link(s) with anchors.
-> Creating 7 pages (0 orphan) and 1 sections
thread 'main' panicked at 'no entry found for key', components/config/src/config/markup.rs:78:33
stack backtrace:
   0:        0x1053a75ff - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hd4fa0e5fd1b0bae0
   1:        0x104e733eb - core::fmt::write::hd6db5571778cce6c
   2:        0x10538873a - std::io::Write::write_fmt::hf56449a28d15bb2b
   3:        0x105390645 - std::panicking::default_hook::{{closure}}::h2d0f2e1e91f96447
   4:        0x105391109 - std::panicking::rust_panic_with_hook::h916e420d7d249caa
   5:        0x1053a79da - std::panicking::begin_panic_handler::{{closure}}::hc926ed443b77e590
   6:        0x1053a7959 - std::sys_common::backtrace::__rust_end_short_backtrace::hf8832c1005df0617
   7:        0x105390985 - _rust_begin_unwind
   8:        0x10561384f - core::panicking::panic_fmt::hedd127944bd71c57
   9:        0x104e6e08b - core::panicking::panic_display::hfb53cabd7daaeed2
  10:        0x10561322c - core::option::expect_failed::hdae270e7e6b89f17
  11:        0x104e1470d - config::config::markup::Markdown::get_highlight_theme_by_name::h806ae2a395ecc82d
  12:        0x10533d05b - site::Site::build::h55620189ec0474ac
  13:        0x104d6f81c - zola::main::he9876c96633a39fe
  14:        0x104d07a66 - std::sys_common::backtrace::__rust_begin_short_backtrace::hd2c498feb33f3837
  15:        0x104d7216b - _main

$ RUST_BACKTRACE=1 zola build
Building site...
Checking all internal links with anchors.
> Successfully checked 0 internal link(s) with anchors.
-> Creating 7 pages (0 orphan) and 1 sections
thread 'main' panicked at 'no entry found for key', components/config/src/config/markup.rs:78:33
stack backtrace:
   0: _rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::panic_display
   3: core::option::expect_failed
   4: config::config::markup::Markdown::get_highlight_theme_by_name
   5: site::Site::build
   6: zola::main
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Step to reproduce

[markdown]
highlight_code = true
highlight_theme = "css"
extra_syntaxes_and_themes = ["extra_highlight_themes"]
highlight_themes_css = [
  { theme = "base16-unikitty-dark", filename = "syntax-theme-dark.css" },
  { theme = "base16-unikitty-light", filename = "syntax-theme-light.css" },
]

$ ls -lh extra_highlight_themes
total 24K
-rw-r--r--  1 arne staff 12K Feb 20 14:22 base16-unikitty-dark.tmTheme
-rw-r--r--  1 arne staff 12K Feb 20 14:23 base16-unikitty-light.tmTheme

[Keats]

That shouldn't be too hard to fix if someone wants to try!