Semantic-ui: why the grid's margin = -1rem

[funkyLover]

when I use grid in my page
the horizontal scrollbar always appear
it make the page get wider than window.width
I find that the margin in css

I can't understand why set margin a negative value?

[jlukic]

Use a padded grid vertically padded or horizontally padded grid variation that preserve the padding on first and last columns.

Grids sit flush with surrounding content by default. Negative margins on the outer grid counteract the spacing between all column. There's no way to adjust padding for first and last columns instead of using negative margins except to add row demarcations.

Notes on negative margins are in the docs, in the grid overview

[funkyLover]

@jlukic thank,
I missed that content in the docs
I should check the docs carefully = =~