Now Experience Framework > Theme Management > Themes. From here, click the New button to create a new theme.Create Theme and Create Stylesheet and add below properties:

{
    "base": {
        "--now-color--neutral": "153,153,153",
        "--now-color--primary": "102,102,102",
        "--now-color--secondary": "153,153,153",
        "--now-color_chrome--brand": "102,102,102"
    },
    "properties": {
        "--now-color_brand--neutral": "153,153,153",
        "--now-color_brand--primary": "102,102,102",
        "--now-color_brand--secondary": "153,153,153"
    }
}

Override the Default Theme:

1. 1. Copy the UX Theme sys_id by right clicking the header and selecting Copy sys_id.
2. 2. In the Application Navigator navigate to sys_properties.list.
3. 3. Create a new system property named: glide.ui.polaris.theme.custom with the Type of string.
4. 4. Enter your copied sys_id into the Value field.
5. 5. Save the record.

Visit below website to generate Stylesheet & Color

1. https://theme.deoprototypes.com/
2. https://redketchup.io/color-picker

Highlighted Properties and usages:

1. –now-unified-nav_menu–background-color – Used for Left navigation
2. –now-color_background–primary – Form Background Color
3. –now-color_surface–neutral-3 – Form Header Menu
4. –now-unified-nav_header–background-color – Main Logo and Header color
5. “–now-font-family”:”papyrus” – For font change