First, change your code into the following:
$value = get_option( 'mytheme_value' );
query_posts ( "meta_key=post_views_count&orderby=meta_value_num&order=DESC&meta_value={$value}");
When you use single quotation marks, php isn’t parsed in the string, and functions (like get_option) will never be called from within a string. Therefore, call the function get_option first, and put the value in a double quoted string (which parses variables during execution).
Then change your other code into the following:
// ADD THE FORM INPUT TO $new_post ARRAY
$new_post = array(
'post_title' => $title,
'post_content' => $description,
'post_category' => array($_POST['cat']), // Usable for custom taxonomies too
'tags_input' => array($tags),
'post_status' => get_option( 'mytheme_status' ),
When you want to call a function inside an array, using whatever it returnes as value, you don’t need to put the function inside single quotes.
Furthermore, if you want to get option values in functions.php, you can simply use the following line:
$mytheme_status = get_option( 'mytheme_status' );
When you still encountering errors after these modifications, and you still get a blank screen, try to find out whats going wrong by looking in your error logs. On a linux environment, the default apache error log file location is /var/log/apache2/error.log or /var/log/httpd/error_log.