creando restricciones en mySQL

[Libras]

Sip pero si te fijas en este codigo:

create table Themes ( ThemeID int primary key, ThemeName varchar(100) );
create table Users ( UserID int primary key, UserName varchar(100), ThemeID int constraint Users_ThemeID_FK references Themes(ThemeID) on delete set null);

Falta una coma en el themeid por eso no te reconoce el constrait :) ademas de que te falta especificar cual es la columna que va a ser FK :P

quedaria:

Código MySQL:

create table Themes ( ThemeID int primary key, ThemeName varchar(100) );
create table Users ( UserID int primary key, UserName varchar(100), ThemeID int, constraint Users_ThemeID_FK foreign key (themeid) references Themes(ThemeID) on delete set null);