You can't change it in the UI yet, but you can change the colors using CSS variables

For example here's catppuccin, you can copy & paste into a browser extension like Stylus

:root {
    --accent: #f2cdcd;
    --accent-fg: var(--accent);
    --bg: #11111b;
    --fg: #cdd6f4;
    --fg-muted: #a6adc8;
    --fg-on-accent: #1e1e2e;
    --divider: #c8c8ff1a;
    --divider-hover: rgba(120, 120, 200, 0.4);
    --panel: #181825;
    --panel-hover: #1e1e2e;
    --panel-elevated: var(--panel-hover);
    --shadow: 0 4px 25px #0000000a;
    --modal-bg: #00000080;
    --scrollbar-handle: #ffffff33;
    --button-bg: #ffffff0d;
    --button-bg-hover: #ffffff1a;
}