Poor style cutomization

[noraj]

Description

hexo-theme-doc/source/style/_doc/vars.scss doesn't allow to

1. configure the main background color
2. use variables when overwriting in source/styles/doc.scss eg $doc-navbar-background-color: $doc-color-darkest !default;

So for 1., one can't customize into a dark theme:

And for 2. one must set color directly $doc-navbar-background-color: #333 !default; loosing the interest of variables.

Technical info

$ node --version
v10.15.3

$ npm --version
6.4.1

$ hexo --version
hexo: 3.9.0
hexo-cli: 1.1.0
os: Linux 5.2.2-arch1-1-ARCH linux x64
http_parser: 2.8.0
node: 10.15.3
v8: 6.8.275.32-node.51
uv: 1.23.2
zlib: 1.2.11
ares: 1.15.0
modules: 64
nghttp2: 1.34.0
napi: 3
openssl: 1.1.0j
icu: 62.1
unicode: 11.0
cldr: 33.1
tz: 2018e