For some reason, the path to the images that make up the buttons, contains the full, absolute URL (with http) on your end.
This is how the original code of my plugin calls these buttons:
.page-scroll-buttons button.one-page-down {background-image:url('../img/spsb-arrow-down.png');}
.page-scroll-buttons button.one-page-up {background-image:url('../img/spsb-arrow-up.png');}
.page-scroll-buttons button.all-the-way-to-top {background-image:url('../img/spsb-arrow-top.png');}
As you can see, it doesn’t contain the domain name or path to your WordPress install.
This is how the code is on your site:
.page-scroll-buttons button.one-page-down {background-image: url(https://kagimedia.com/wp-content/plugins/smooth-page-scroll-updown-buttons/assets/css/../img/spsb-arrow-down.png);}
.page-scroll-buttons button.one-page-up {background-image: url(https://kagimedia.com/wp-content/plugins/smooth-page-scroll-updown-buttons/assets/css/../img/spsb-arrow-up.png);}
.page-scroll-buttons button.all-the-way-to-top {background-image: url(https://kagimedia.com/wp-content/plugins/smooth-page-scroll-updown-buttons/assets/css/../img/spsb-arrow-top.png);}
That appears to include the entire path, starting with https://kagimedia.com/, and that is what’s causing the mixed content warnings.
I can’t say if this is something you changed yourself in the code, or if that’s coming from a plugin (perhaps a minification plugin?).
If you changed it yourself, make sure it contains https://kagimedia.com/ (instead of https://kagimedia.com/).
If a plugin changed this, you’ll need to check which plugin that is, and if you can change the settings in there.
