Yes there is a way to customize themes and create your own
https://react.semantic-ui.com/theming/
Basically just follow instructions from the docs link, install semantic-ui-less, which are not compiled Less files for the Semantic-ui CSS, craco-less which is just a path plugin for CRA configured Webpack so your Less compiler can find files properly without ejecting your CRA, move the files to created folder, modify theme.config, start app and you are ready to customize your own theme, although I didn't find some kind of API for variables on Semantic-ui docs, so I had to browse semantic-ui-less package files for them.
One problem I encountered was that I tried changing the theme for all components in theme.config and it was throwing errors about missing fonts and variables. Don't do this, it's not gonna work, check the Semantic-ui docs about theming, there is a note about it
Themes are per component, so although, for example, material themes are available for menu, button, and site, changing all values to "material" will produce an error for components which are not included in that theme.
https://semantic-ui.com/usage/theming.html#browsing-for-themes