Yes, for function overrides you can call through to the "base" function as it says in the documentation:
When overriding functions, you should call through to the _base function as much as possible.
For theme layers it's similar, you should call through to the parent function. It can be a bit complex, especially if you want to change something that the parent function outputs. Generally you should just change variables. I can't think of a great example right now, but let's pretend the default CSS function is like this:
function css()
{
$this->output('<link rel="stylesheet" href="'.$this->cssfile.'">');
}
If you want to output an extra CSS file you would do:
function css()
{
parent::css();
$this->output('<link rel="stylesheet" href="newfile/css">');
}
Or if you want to change what CSS file is output you would do something like this in your plugin:
function css()
{
$this->cssfile = 'newfile.css';
parent::css();
}
