Hi Annabelle

I have a suspicion as to why this is happening - are you using a standard theme for it or have you developed a child theme with your changes to your CSS? If you aren't on a child theme or are using a theme that doesn't have the addition to add CSS as an override, then whenever you do an update to the theme, it will scrap any changes to your CSS and revert back to the original style sheet on the theme template.

Although, I only work on WordPress so this may not be the case if you're on something else.