"The Cariboo Café" is a short story written by Helena María Viramontes, which is set in the Cariboo Café mentioned in the title.
With regard to the tone of this story, you might want to explain that it is overall written in a very dark and somber tone, highlighting the horrible experiences that some migrants are facing.
With regard to the theme of the story, I would firstly like to point out that there are a several themes in this story, but the main theme is undoubtedly relates to the challenges of immigration. The short story is told in three different parts, each part focusing on a different person, but they are all interlinked not only by the fact that the people are visiting the Cariboo Café, but also by the fact that each part is somehow dealing with immigration or aspects of immigration.
For example, in the first part we meet Sonya and Macky and we find out straight away that they are migrant children: "they arrived in the secrecy of night, as displaced people often do." In this part, we read about Sonya's new life and how she has to look after her brother Macky. The tone of this part is rather somber, as it clearly shows us the hardship of Sonya's life, which doesn't display the usual happiness one would expect during childhood.
The second part is centered around the owner of the café, and the reader sees gets an insight into the owner's day at work. For example, we read that the owner is dealing with the police and three migrants, who had tried to hide in his café.
The third part brings part one and part two together, leading to the finale of the story. Again, immigration is an important theme, as the lady, who has abducted Sonya and Macky, is clearly severely impacted by traumatic experiences from her past in her home country. We can see that because she is completely deluded: she believes that Macky is her son Geraldo.