Where is the class "placeholder" defined? According to the function:

function drupal_placeholder($text) {
  return '&lt;em class="placeholder"&gt;' . check_plain($text) . '&lt;/em&gt;';
}


It would return the HTML code with class "placecholder".
I had searched the word "placeholder" in the drupal 7 project about 4,800 files. I expected to find the default CSS definition of class placeholder e.g. em.placeholder{} or .placehoder{}. But I can't.

I can only find some files with html ID attribute #placeholder{} was defined.

Some of the files:
 drupal/modules/color/color-rtl.css 
 drupal/modules/color/color.css 
 drupal/sites/all/module/ctools.css.stylizer.css


Because I want to modify class with my style globally. Please advise if any default definition of class placeholder in drupal 7.
The only uses of the placeholder class in core are (filename, followed by line number, follby the line):

modules/dashboard/dashboard.js:35:        if ($('.placeholder', this).length == 0) {
modules/dashboard/dashboard.js:38:        $('.placeholder', this).html(empty_text);
modules/dashboard/dashboard.js:41:        $('.placeholder', this).remove();
modules/system/system.admin.css:264:.exposed-filters .current-filters .placeholder {


and also a few in jquery.ui that you can ignore:

misc/ui/jquery.ui.draggable.min.js:40
misc/ui/jquery.ui.sortable.min.js:26
misc/ui/jquery.ui.sortable.min.js:27
misc/ui/jquery.ui.sortable.min.js:29
misc/ui/jquery.ui.sortable.min.js:39
misc/ui/jquery.ui.sortable.min.js:40
misc/ui/jquery.ui.sortable.min.js:43
misc/ui/jquery.ui.sortable.min.js:55
misc/ui/jquery.ui.sortable.min.js:56
misc/ui/jquery.ui.sortable.min.js:59
misc/ui/jquery.ui.sortable.min.js:60


You can safely ignore all this and just implement your own css for the .placeholder class in your theme.