If you want to edit the style of the Salient theme beyond what is achievable in the theme options, then you will probably need to edit some CSS and possibly some php files.
You can edit the CSS by using the custom CSS box in the general theme options and for a few smaller CSS edits this is not a problem as these edits are stored in the DB and when you update the master theme those settings should remain intact. If you don’t know much CSS you can use something like CSS HERO to update live in the browser.
If however you will be editing any of the php files you should add a child theme, so that you can still update the master Salient theme without effecting the customisation work you have done.
If you have FTP access to the server it is quite easy to do. Just add a folder, called something like salient-child, into the wp-content>themes folder so it sits next to your master Salient theme folder. Within this folder you need to add a new file called style.css .
In this style.css file in your child theme folder you need to add the following;
Theme Name: Salient child theme
Then go to your WP admin panel and under themes, activate your new child theme. If you have done it right, you will see no changes on the front end as the child theme has no content yet. Once you add new CSS to your child theme it will override the master Salient theme CSS.
Now with the Salient child theme active, next time an update for Salient is released you can update the master Salient theme files and your Salient child theme files and edits will remain intact.