feat: Add built-in theme customization

Hello,

I'd like to request a theme customization feature to allow users to easily switch and personalize themes without relying on third-party browser extensions.

Currently, users who want to customize the appearance of the app must rely on external tools like the Stylus Firefox extension to manually inject custom CSS into the document. A built-in theme system would provide a more seamless and accessible experience for all users, regardless of their browser or technical setup.

For example, I use this CSS code to mimic the appearance of Open WebUI:

 html {
    color-scheme: dark !important;
    --app-bg: #0f0f0f !important;
    --app-fg: #f1f1f1 !important;
    --app-border: color-mix(in oklab, var(--app-fg) 1.5%, transparent) !important;
    --app-divider: color-mix(in oklab, var(--app-fg) 0.875%, transparent) !important;
}

aside > :nth-child(2) {
    & > a {
        margin: 1px 0px 0px 1px;
        gap: 0.75rem !important;
        font-weight: normal !important;
    }
   
    & > a > img {
        width: 20px;
        height: 20px;
        margin-left: -6px;
    }
}

main {
    --app-bg: #161616;
    --app-border: color-mix(in oklab, var(--app-fg) 5%, transparent);
    background-color: var(--app-bg);
}

#main-col > div {
    --app-bg: #161616;
    --app-border: color-mix(in oklab, var(--app-fg) 7.5%, transparent);
    background-color: var(--app-bg);
}

.app-surface:has(.chat-editor-mount) {
    --app-bg: #1c1c1c;
    --app-border: transparent;
    position: relative;
    background-color: var(--app-bg);

    & span.truncate {
        max-width: none !important;
    }
   
    &:focus-within, &:hover {
        --app-border: color-mix(in oklab, var(--app-fg) 7.5%, transparent);
    }
    
    &:after {
        content: '';
        position: absolute;
        bottom: 100%;
        right: 100px;
        width: 40px;
        height: 35px;
        background: url('https://cdn.jsdelivr.net/gh/homarr-labs/dashboard-icons/png/clawd.png');
        background-size: cover;
        image-rendering: pixelated;
    }
}

.resize-handle, .git-resize-handle {
    background: none !important;
}

div.pane-content > div > div > div > .max-w-full {
    width: 80% !important;

    &.flex-col {
        zoom: 0.95;
    }
   
    & .not-prose:not(.group\/table) {
        background-color: black !important;
    }

    & .chat-editor-mount {
        & ol {
            list-style: decimal;
        }
       
        & ul {
            list-style: disc;
        }
    }
}

button.max-w-32 {
    max-width: none !important;
}

div.svelte-1fbx6nu {
    width: auto !important;
}

div.svelte-1ieqiib .text-left {
    font-weight: normal !important;
}

div.svelte-1ieqiib .text-xs {
    font-size: 13px !important;
}

div.svelte-1onbaf .select-none.text-gray-400 {
    width: 29px;
}

Thank you for considering this feature. It would greatly improve the user experience and accessibility of the app.

Thank you for your work and I wish you all a great day,
Joshua Krimmer

The issue does not identify files, tests, or an entry point. Start by reviewing the existing appearance and styling architecture, then compare it with the supplied CSS customization example. Done should mean users can switch and personalize themes within the app without relying on a browser extension.