Can not make changes in server-side


last week I downloaded ‘Pico CMS’ for Nextcloud as app. After that I created a website with the app, using the default theme/design and as template ‘sample_pico’.
Today I want to make some changes server-side, like changing the primary-color, logo, adding an hover-effect to some elements and so on.

To my understanding (correct me If I have missunderstood something):
At first: my cms_pico path is: /var/www/nextcloud/apps/cms_pico

So the path to the config-file is: <cms_pico_path>/appdata/config
I renamed the config-template to config.yml.

Also all themes are located at: <cms_pico_path>/appdata_public/themes, with ‘j753is5shZ’ as default-theme.

If I edit (f.e.) css/style.css and save it and then reload the website, it does not use the changes.
And if I open the css-file via browser-developertools I get the unchanged css-file. Whyever.

Does anyone know what the reason for is?