Theme variables are missing in docs

[Mister-Hope]

Description

There are a lot of theme variables which is actually working, but can not be found in docs, we cannot expect every user to read source code themselves.

An example is attributeBackgroundColorOdd and attributeBackgroundColorEven (for er chart), can only be found at source code, and a related bug report at https://github.com/orgs/vuepress-theme-hope/discussions/3433

Suggested Solutions

I would recommend adding theme in docs, and I would like to submit a pr.

The only question is that shall I add them at the bottom of chart page, or inside config section about themeVariables?