Switched back from kanboard to Vikunja.

/etc/kanboard/config.php

/usr/share/kanboard

this is where index.php is!!!

/var/lib/kanboard/

this is where data, files and plugins are

Working nginx config file for kanboard installed from official debian package and served under a /kanboard subdirectory:

server {
        listen          80;
        listen          [::]:80;
 
        server_name     orion.sysinfo.ro;
        client_max_body_size 32M;
 
        root /usr/share;
        index   index.php;
 
        location /kanboard {
                try_files $uri $uri/ /kanboard/index.php$is_args$args;
 
 
                location ~ \.php$ {
                        try_files $uri =404;
                        fastcgi_split_path_info ^(.+\.php)(/.+)$;
                        fastcgi_pass unix:/var/run/php/php-fpm.sock;
                        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
                        fastcgi_index index.php;
                        include fastcgi_params;
                }
 
                location ~* ^.+\.(log|sqlite)$ {
                        return 404;
                }
 
                location ~ /\.ht {
                        return 404;
                }
 
                location ~* ^.+\.(ico|jpg|gif|png|css|js|svg|eot|ttf|woff|woff2|otf)$ {
                        log_not_found off;
                        expires 7d;
                        etag on;
                }
 
 
        }
}

An interesting way to do this is to use Dark Reader and copy the CSS rules to kanboard as a custom stylesheet. This way, the extension isn't needed.
The current CSS:

/* Common theme */
html,
body,
:not([style*="background-color:"]):not(iframe) {
    background-color: rgb(16, 20, 23) !important;
}
html,
body,
:not([style*="color:"]),
.sr-reader *:not([class*='sr-pivot']) {
    color: rgb(167, 158, 139) !important;
}
h1:not([style*="color:"]),
h2:not([style*="color:"]),
h3:not([style*="color:"]),
h4:not([style*="color:"]),
h5:not([style*="color:"]),
h6:not([style*="color:"]) {
    color: rgb(164, 211, 233) !important;
}
cite:not([style*="color:"]) {
    color: rgb(128, 204, 148) !important;
}
input:not([style*="background-color:"]),
textarea:not([style*="background-color:"]),
button:not([style*="background-color:"]),
[role="button"] {
    background-color: rgb(32, 48, 64) !important;
}
input:not([style*="background-color:"]):hover,
textarea:not([style*="background-color:"]):hover,
button:not([style*="background-color:"]):hover,
[role="button"]:hover {
    background-color: rgb(30, 54, 78) !important;
}
input:not([style*="background-color:"]):active, input:not([style*="background-color:"]):focus,
textarea:not([style*="background-color:"]):active, textarea:not([style*="background-color:"]):focus,
button:not([style*="background-color:"]):active, button:not([style*="background-color:"]):focus,
[role="button"]:active, [role="button"]:focus {
    background-color: rgb(29, 56, 83) !important;
}
a:not([style*="color:"]) {
    color: rgb(128, 182, 204) !important;
}
a:not([style*="color:"]):hover {
    color: rgb(142, 193, 217) !important;
}
a:not([style*="color:"]):active, a:not([style*="color:"]):focus {
    color: rgb(155, 203, 230) !important;
}
:not([style*="border-color:"]),
::before,
::after {
    border-color: rgb(80, 115, 134) !important;
}
div:empty,
.sr-reader *,
.sr-backdrop {
    background-color: rgba(16, 20, 23, 0.5) !important;
}
input::placeholder,
textarea::placeholder {
    color: rgba(167, 158, 139, 0.5) !important;
}
input:not([style*="background-image:"]),
textarea:not([style*="background-image:"]) {
    background-image: none !important;
}

See Kanboard docs.

Dark theme for Kanboard