If you use the dark mode plugin, here is some updated code provided by our friendly AI that fixed the inverted pictures. I am still going to tweak this plugin some more to try and clean up the colors of the fonts to make it a more black and white theme. The colors tend to make it look a little busy.

The file you need to edit is located in the Pluginst/darkmode folder and is named: darkmode.php You need to completely replace the code block starting with: /**
* On charge la minification avant
**/
function darkmode_head()
{

Here's the updated code block that will fix the inverted images.

function darkmode_head()
{
    $plugin = 'darkmode';
    # Lecture des données
    $data = flatDB::readEntry('plugin', $plugin);
    $config = flatDB::readEntry('config', 'config');
    $out = '';
    $assets = HTML_PLUGIN_DIR . $plugin . DS . 'assets' . DS;

    if ($data[$plugin . 'state']) {
        $out .= '	<link rel="stylesheet" type="text/css" href="' . $assets . 'darkmode.min.css?ver=1.5.7" />' . PHP_EOL;
        // Add custom CSS to prevent image inversion
        $out .= '	<style>
                        /* Target images with high specificity */
                        html body.darkmode--activated img,
                        html body.darkmode--activated img.img-fluid,
                        html body.darkmode--activated img.img-thumbnail,
                        html body.darkmode--activated .darkmode-ignore,
                        html body.darkmode--activated picture,
                        html body.darkmode--activated video,
                        html body.darkmode--activated iframe,
                        html body.darkmode--activated .darkmode-image-wrapper {
                            isolation: isolate !important;
                            mix-blend-mode: normal !important;
                            background-blend-mode: normal !important;
                            display: inline-block !important;
                            position: relative !important;
                            z-index: 1 !important;
                            -webkit-isolation: isolate !important;
                            -webkit-mix-blend-mode: normal !important;
                            /* Counteract inversion caused by mix-blend-mode: difference */
                            filter: invert(1) !important;
                        }
                        /* Isolate parent elements */
                        html body.darkmode--activated .post-content,
                        html body.darkmode--activated .image-wrapper,
                        html body.darkmode--activated .container,
                        html body.darkmode--activated .row,
                        html body.darkmode--activated .col,
                        html body.darkmode--activated .card,
                        html body.darkmode--activated .post,
                        html body.darkmode--activated .thread {
                            isolation: isolate !important;
                            mix-blend-mode: normal !important;
                            background-blend-mode: normal !important;
                            position: relative !important;
                            z-index: 1 !important;
                            -webkit-isolation: isolate !important;
                            -webkit-mix-blend-mode: normal !important;
                        }
                        /* Handle background images */
                        html body.darkmode--activated [style*="background-image"],
                        html body.darkmode--activated [style*="background"],
                        html body.darkmode--activated .darkmode-ignore [style*="background-image"],
                        html body.darkmode--activated .darkmode-ignore [style*="background"] {
                            isolation: isolate !important;
                            mix-blend-mode: normal !important;
                            background-blend-mode: normal !important;
                            position: relative !important;
                            z-index: 1 !important;
                            -webkit-isolation: isolate !important;
                            -webkit-mix-blend-mode: normal !important;
                        }
                        /* Target common Flatboard classes */
                        html body.darkmode--activated .post-image,
                        html body.darkmode--activated .avatar,
                        html body.darkmode--activated .attachment {
                            isolation: isolate !important;
                            mix-blend-mode: normal !important;
                            background-blend-mode: normal !important;
                            position: relative !important;
                            z-index: 1 !important;
                            -webkit-isolation: isolate !important;
                            -webkit-mix-blend-mode: normal !important;
                        }
                        /* Ensure the darkmode-layer doesn\'t interfere */
                        .darkmode-layer {
                            pointer-events: none !important;
                            z-index: 499 !important;
                        }
                    </style>' . PHP_EOL;
    }
    return $out;
}

Thanks minim, I just updated the plugin by integrating your css into the minifier file.

After Install the darkmode plugin a big blue arrow appears in the left Side.
But only in dark mode.
How can I repair this?