Fix dark mode for Tokenizer input text
Summary:
Replace hardcoded text color `#333` for tokenizer input text with variable `{$greytext}` which feels close enough.
Refs T15056
Test Plan:
* Go to http://phorge.localhost/settings/panel/display/ and set `Accessibility` to `Dark Mode (Experimental)`
* Go to http://phorge.localhost/maniphest/task/edit/1/, type text in the "Tags" or "Subscribers" field, check if you can read the text you entered
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15056
Differential Revision: https://we.phorge.it/D26213