How can I use this php empty function to properly display CSS?

I made a page where users can input a Hex Code for different colors they want to change on their page. It all works fine, but I would like to make it so if the box the user is inputting to is empty, then display the default CSS. Otherwise, display a custom one.

I'll use an example:

The users choice gets sent to a form validation:

$this->form_validation->set_rules('channelNameColor',
                    lang('user_themes_channel_name_color'),
                    'trim|strip_tags|xss_clean|min_length[6]|max_length[6]');

Then it eventually gets passed to a file where it can be used in CSS:

It is declared here:

$channelNameColor = $theme['channelNameColor'];

Then down below all those declarations it gets put in as CSS:

.channelNameBar h1{
font-size:20px;
color:#<?=$channelNameColor?>;
float: left;
}

I would like to do something like this:

<?php
if (empty($channelNameColor)){
     echo ".channelNameBar h1{";
     echo "font-size:20px;";
     echo "color:#FFFFFF;";
     echo "float: left;";
     echo "}";
}
else
 {
     echo ".channelNameBar h1{";
     echo "font-size:20px;";
     echo "color:#$channelNameColor;";
     echo "float: left;";
     echo "}";
 }
?>

But it is not recognizing $channelNameColor in: if (empty($channelNameColor)){

Why is it not recognizing it there, but it recognizes it fine when I use it in the css? Sorry if this is a very simple question to answer, I'm pretty new to PHP