How to find the hightlight CSS of task?

Gantt
#1

If I click the task whose edge looked not very clearly.

#2

Hello,
You can use the gantt_selected selector for the selected tasks. To affect the task bars in the timeline, you need to use the gantt_task_line selector:

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You can read more about selectors in the following article:
https://docs.dhtmlx.com/gantt/desktop__css_overview.html