SQLITE the datatype is ‘TEXT’
After testing, i can put way more than 255 chars into field no issue.
as far as i can tell, nothing in Kanboards Core is limiting this to 255 chars, for SQLITE.
FYI, 858330 was the max amount Chars I could have it save:

It’s MySql and postgress where 255 chars is clearly defined as the limit.
for a breif time, the settings table value was also defined as varchar(255), but was changed to type ‘Text’, most likely not long after the CustomCss option was added:
so, yes, you could do the same in the core for the user_has_metadata value, or you could include a schema with this plugin that makes the modification. There are other options, but it seems like we are hell bent on modifying databases, so there you go.
As for SQLITE, if you have a limit of 255, i’d say look at your setup and what you have going on, maybe you set a limit.
edit:
I’ll include a couple other options that do not involve modifying the databse, because, well…why not:
1.) input css into the field, instead of saving directly to the DB, output a file with the css, and only save the filename into the user_has_metadata field.
2.) Don’t use that table, use the settings table, it’s already setup for you