Customizing and Styling “Post Pagination”?

I’m trying to create a custom style for my post Pagination, i.e. the tag. And both plug-ins (such as WP-Paginate) and non-plug-in code solutions require finding the and replacing your existing previous_posts_link() and next_posts_link() code block. Where is this located in this theme?

And although it has a custom CSS field, I shouldn’t be adding this here, is that correct? I’m a bit confused where I should be modifying this within this theme.

Here is some non-plugin code I would like to add to my posts (maybe pages too), do you know where I should be adding it so it wont be wiped out by a theme update?

To begin your nextpage “paginated links” makeover, add the following to your custom_functions.php file contents:

function custom_nextpage_links($defaults) {
$args = array(
‘before’ => ” . __(‘Sections —’),
‘after’ => ”,
);
$r = wp_parse_args($args, $defaults);
return $r;
}
add_filter(‘wp_link_pages_args’,’custom_nextpage_links’);
For the color and style, add this to your custom.css file contents:

.custom .my-paginated-posts {
font-size:10px; font-weight:normal; font-family:arial; }

.custom .my-paginated-posts p { font-size:1.4em; }

.custom .my-paginated-posts p a {
background:#E8E8E8; color:#000; margin-left:0.6em; margin-right:0.6em;
padding:0.5em 0.7em; text-decoration:none; }

.custom .my-paginated-posts a:hover { color:#fff; background:#7F1315; }