[Feature] Improve darkMode option docs

[DavidMarquezF]

What problem does this feature solve?

Right now it is not clear how to use darkMode from the docs. The values that can be assigned are explained but not how it will behave.

I think it may be because it is not clear if setting darkMode to true will switch to the dark theme or it will do something else, since everywhere else in the docs it says that in order to use the dark theme you need to init the chart with that theme.

Furthermore, darkMode auto just checks the chart's backgroundColor property, but by default it is transparent (it will always guess it is light unless the user specifies a color for background?). From the docs you might get the impression that the chart checks the background visible by the user, meaning that even though the backgroundColor property is transparent, the chart surely has a background Color from other html elements in the web. I think it should be made more clear.

What does the proposed API look like?

Clarify the darkMode docs answering the questions stated above

[github-actions[bot]]

This issue has been automatically marked as stale because it did not have recent activity. It will be closed in 7 days if no further activity occurs. If you wish not to mark it as stale, please leave a comment in this issue.

[github-actions[bot]]

This issue has been automatically closed because it did not have recent activity. If this remains to be a problem with the latest version of Apache ECharts, please open a new issue and link this to it. Thanks!