[v2] use primary color for highlighted items in table of contents

[banli17]

v2 use primary color for highlighted items in table of contents

[wgao19]

I think highlighting table of content isn't implemented in v2 yet. There is a related v1 PR #1524.
Would you like to work on this?

[banli17]

i make it local，but i dont know put js where, i put css and other in website dir, but if js put in theme dir , it's strange

[wgao19]

I would create a component under packages/docusaurus-theme-classic/src/theme and use it in packages/docusaurus-theme-classic/src/theme/DocLegacyItem/, not sure if @endiliey or @yangshun would suggest anywhere else?

[yangshun]

Correction - this issue is not related to #1871 (got misled by #1871's inaccurate description). #1896 is the fix in the right direction.