Darken your Swagger theme and improve your user experience by using the codes below

Program.cs

// Add this middleware to access static files
app.UseStaticFiles();

// Inject the CSS file into the Swagger setup
app.UseSwaggerUI(setupAction => setupAction.InjectStylesheet("/css/swagger.css"));

swagger.css

:root {
    --back: #191919;
    --sections: #323130;
    --secondary-back: #0d101b88;
    --text: #fff;
    --primary: #69bbde;
    --dark-shadow: #11142288;
    --ligth-shadow: #084e8888;
    --prop-type: #61afed;
    --primitive: #b03f34;
}

* {
    color: var(--text) !important;
}

body {
    background-color: var(--back);
}

.topbar {
    box-shadow: 0 0 5px var(--ligth-shadow);
}
.info {
    box-shadow: 0 0 5px 2px var(--ligth-shadow);
    padding: 10px 30px !important;
    border-radius: 5px;
}
.version-stamp {
    background-color: var(--primary) !important;
}
.scheme-container {
    background-color: var(--back) !important;
    box-shadow: none!important;
}

input,
textarea {
    background-color: var(--back) !important;
    border: 1px solid var(--text) !important;
}

select {
    background-color: var(--back) !important;
    border: 2px solid var(--primary) !important;
}

select > option {
    background-color: var(--back) !important;
}

.btn {
    border: 2px solid var(--primary) !important;
}

code {
    color: var(--primary) !important;
    background-color: var(--secondary-back);
    padding: 3px;
    border-radius: 4px;
}

pre > code {
    color: var(--text) !important;
    background-color: transparent;
}

.opblock-tag {
    border: none !important;
}
.opblock-section-header {
    background-color: var(--back) !important;
    box-shadow: 0 5px 5px var(--dark-shadow) !important;
}
.opblock-tag-section {
    margin-top: 10px;
    border-radius: 4px;
    padding: 5px 10px;
    background-color: var(--sections);
}

.modal-ux-content {
    background-color: var(--back) !important;
}
.modal-ux-header {
    background-color: var(--sections) !important;
}

section.models {
    background-color: var(--sections);
}

.model-container {
    border: none;
    box-shadow: 0 0px 5px 3px var(--dark-shadow);
}

.property-row {
    background-color: var(--secondary-back);
    display: flex;
    align-items: center;
    padding: 5px;
    margin: 5px;
    border-radius: 4px;
}

.response {
    border-top: 1px solid var(--text);
}

span.prop-type, div.parameter__type {
    color: var(--prop-type) !important;
}
span.primitive {
    color: var(--primitive) !important;
}