template: Switch to Solarized for syntax highlighting

The default theme doesn't really work well in dark mode, and even in
light mode it "screams" a lot.

The Solarized color scheme, on the other hand, looks better in light
mode, and has a dark mode as well.

We can use the @import rule to determine whether to load the light or
dark Solarized scheme depending on the browser settings.

Fixes: #101
3 jobs for highlight-theme in 1 minute and 45 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Check
passed flake8 #1492350

00:01:44

failed mypy #1492351
allowed to fail

00:01:39

passed reuse #1492352

00:01:44

 
Name Stage Failure
failed
mypy Check
gidocgen/gir/parser.py:986: error: Argument "get_type" to "GType" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:1008: error: Argument "name" to "Union" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:1008: error: Argument "symbol_prefix" to "Union" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:1008: error: Argument "ctype" to "Union" has incompatible type "Optional[str]"; expected "str"
gidocgen/gir/parser.py:1010: error: Argument 1 to "set_version" of "GIRElement" has incompatible type "Optional[str]"; expected "str"
gidocgen/utils.py:823: error: Need type annotation for 'found_programs' (hint: "found_programs: Dict[<type>, <type>] = ...")
Found 218 errors in 3 files (checked 17 source files)
Cleaning up file based variables
ERROR: Job failed: exit code 1