Why wireframe thickness changes in edge mode? I didn't change standard theme
But in edge mode, wireframe changes it's thickness. 
I don't want that to happen. I need one thickness for all modes. How can I do that?
Swapping between the different edit modes changes the look to show you what you're editing. Vertex select makes the points thicker, face select will highlight faces, and in your case, edge select makes the edges thicker.

It's purely UI based, the actual thickness of your edges is not changing. Renders won't look any different.

To display the same thickness, use the Viewport Overlays menu.