Hi @gh X
We are unable to change the color and line thickness of active cells.
For your request, it's recommended to upvote the threads in Microsoft Feedback forum:
make the active cell's thick border even thicker
Make Selected Active Cell customizable so it's easier to see
But you may change the gridline color, check whether the active cells border is much easier to see.
Click File menu > Options > Advanced, scroll down to Display section, change Gridline color.
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