Customes theme and dark-mode for goindex

[churongcon]

Hello I have added 2 pull requests to add Customes theme and dark-mode for goindex
Main Color
red pink purple deep-purple indigo blue light-blue cyan teal green light-green lime yellow amber orange deep-orange brown grey blue-grey

Accent Color
red pink purple deep-purple indigo blue light-blue cyan teal green light-green lime yellow amber orange deep-orange

darkmode: true/false

[yanzai]

Done in version _4.28 . It seems that your pull request code missed some variables, so I made some changes and recommited.