Can own skin modify content in program folder

[Termi]

I am changing a skin and have an issue.
I want to get rid of the labels for username and password on the login screen and replace it with the html tag "placeholder" in the text and password input fields.
I made the labels invisible by changing css display:none

To insert the placeholder tag into the text input I have to change program/include/rcmail_output_html (add the tag) and program/lib/Roundcube/html.php (allow the tag).

It works when I do this in the original roundcube folders but that's not the idea of skins.
I tried to do this in skins/myskinname/program/include/rcmail_output_html and skins/myskinname/program/lib/Roundcube/html.php but these files are not regarded.

Does this work as designed or am I missing something?
Is there a way to achieve my goal within the skins folder?

[JohnDoh]

you cannot do it from the skins folder but using a plugin you could override the default handler for the login form and write your own.

[SKaero]

Or you can add the placeholder text with Javascript which can be done from the skins folder.