Child Themes and there proper usage

I’m trying to figure out what the proper usage of child themes with prebuilt/prebought themes is. I purchased a premium theme and Im trying to edit the lines of code within the template pages of the prebuilt theme. I would like to know, is it possible to create a single child theme file(.css .sass or php or whatever) file and include all of my @import edits in that single page?
Right now I’ve tried to change my child themes templates in .css, however it limits me to one @import(“”) line per style sheet.

I just want all of my template edits in one logical place(preferably one file).

This is where I was pre this question

/* Theme customization-1-decrease distance between checkbox options in contactform 7
-------------------------------------------------------------- */
@import url(".../######.##/wp-content/plugins/contact-form-7/includes/css/styles.css");

.wpcf7-list-item {
	margin-left: none !important;
}

/* Theme customization-2-add columns to contact form 7
-------------------------------------------------------------- */
@import url(".../#######.##/wp-content/plugins/contact-form-7/admin/css/styles.css");

.column-left{ float: left; width: 33%; }
.column-right{ float: right; width: 33%; }
.column-center{ display: inline-block; width: 33%; }

/* Theme customization-3
-------------------------------------------------------------- */
@import url("");

It would help if I could see an example of the structure of a post modified child theme. Preferably one built from a semi-complex premium template. Thanks in advance