Have you ever edited any of your theme files directly to make changes? If you edited the theme files directly, then it’s expected that every time you update the theme to the latest version your changes will be overwritten.
If your changes are CSS only, then the recommended route is to use a custom CSS plugin for your changes.
If you need to also make changes to other theme files, then the recommended route is a child theme.
Both methods protect your changes from being overwritten.
Your subject mentions a theme update, but in your post you said you didn’t do any theme updates, so I’m not clear on that. You’re currently running version 1.0.5 of Sela, which is not the latest (1.0.11).
Custom CSS Option
An easy way to add custom CSS is to install the Jetpack plugin and activate the Custom CSS module. You’ll then add your custom CSS in the new stylesheet editor that’ll appear in your dashboard, under Appearance > Edit CSS.
You could install a standalone custom CSS plugin as an alternative.
Child Theme Option
Making a child theme means your changes won’t be overwritten when you update the theme. If you’re new to child themes, you can explore these guides:
https://codex.www.ads-software.com/Child_Themes
https://www.smashingmagazine.com/2016/01/create-customize-wordpress-child-theme/
https://op111.net/53/
