Try putting this code on your seattle.html
page and then publish it.
But don't forget to make the copy of that seattle.html & master page before you make any changes.
Apply that copy of master page to your site, by setting that master page as default, instead of directly making the changes to the original.
Try making & applying copy of that seattle html master page instead,
.ms-core-listMenu-item, .ms-core-listMenu-item:link, .ms-core-listMenu-item:visited, .ms-tv-item:link, .ms-tv-item:visited, .ms-tv-header:link, .ms-tv-header:visited {
color: #666666;
text-decoration: none;
display: none;
}
site settings
>Master pages and page layouts
?site settings>Master page
). Download it, modify it, and upload it back. I highly suggest you read some tutorial before you actually do it. (e.g. this one) BTW, you can modify the seattle.html instead of seattle.master but it is not easy to discuss this way. You can search for "html master page"