This theme uses a framework called Bootstrap. This means there are predefined structures set for creating the layout of the theme. Example:
<div class="container">
<div class="row">
<div class="col-lg-12">
your content goes in this
</div>
</div>
</div>
The bootstrap framework also allows for columns. The col-lg-12 means one column (full width). You can read more about this here:
https://getbootstrap.com/css/#grid
IT’s a lot of reading, but normally it’s best to leave the bootstrap CSS alone. However, if you need a certain look and layout that requires modifying the Bootstrap code, then you can do it.
The classes you modified are global, which means they are used in other parts of the theme and the site. Basically making the .container as 100% means that all containers with that class will now be 100% (fluid).
Bootstrap does have a class for making the structure fluid, but it would mean copying over several theme files into your child theme and changing all instances (or the ones you want), from .container to .container-fluid
<div class="container-fluid">
When I say risky, I mean that what you change is going to affect other areas that uses those classes and containers. Means every page in this site will be full width (fluid) to the browser window. The column that uses the class=”col-lg-12″ will now have a padding of 0 pixels in space removed from all areas of the site that uses that class.