atomico-css-styling

name	atomico-css-styling
description	Use the css tagged template literal and adoptedStyleSheets for styling Atomico components. Triggers when the user needs to style web components, use CSS custom properties, create themeable components, or understand how Atomico manages stylesheets. Do NOT trigger for external CSS frameworks unless combined with Atomico's css utility.
license	MIT
compatibility	Atomico >=1.79, browsers with adoptedStyleSheets support
metadata	{"category":"core","priority":"medium"}

CSS & Styling

css Tagged Template

Creates and caches CSSStyleSheet objects using the browser's adoptedStyleSheets API for optimal performance.

Basic Usage

import { c, css } from "atomico";

const MyComponent = c(
    () => (
        <host shadowDom>
            <h1>Styled component</h1>
        </host>
    ),
    {
        styles: css`
            :host {
                display: block;
                padding: 1rem;
                font-family: system-ui;
            }
            h1 {
                color: #333;
                margin: 0;
            }
        `
    }
);

How It Works

1. css receives a template literal and produces a CSSStyleSheet
2. Sheets are cached by CSS text — identical styles share one instance
3. Sheets are applied to the component's shadowRoot.adoptedStyleSheets
4. Styles are only applied after the first non-suspended render

Interpolation

const primaryColor = "#3b82f6";

const styles = css`
    :host {
        --primary: ${primaryColor};
        color: var(--primary);
    }
`;

Multiple Style Sheets

The styles option can be an array for composing shared styles:

const baseStyles = css`
    :host { display: block; box-sizing: border-box; }
`;

const themeStyles = css`
    :host { --bg: #fff; --fg: #111; }
`;

const MyComponent = c(
    () => <host shadowDom>content</host>,
    {
        styles: [baseStyles, themeStyles, css`
            h1 { color: var(--fg); }
        `]
    }
);

---

CSS Custom Properties (Theming)

Custom properties pierce shadow DOM boundaries, making them the primary mechanism for theming Atomico components.

Define in Component

const MyCard = c(
    () => <host shadowDom><slot /></host>,
    {
        styles: css`
            :host {
                --card-bg: #f0f0f9;
                --card-border: #dcdce1;
                --card-radius: 0.5rem;
            }
            :host([active]) {
                --card-bg: #a3ebd4;
                --card-border: #6ee2c9;
            }
            :host {
                background: var(--card-bg);
                border: 1px solid var(--card-border);
                border-radius: var(--card-radius);
                padding: 1rem;
            }
        `
    }
);

Override from Outside

/* Page CSS can override custom properties */
my-card {
    --card-bg: #1a1a2e;
    --card-border: #16213e;
}

---

Common Patterns

:host Selector

/* Default state */
:host { display: block; }

/* Attribute-based state */
:host([active]) { background: green; }
:host([disabled]) { opacity: 0.5; pointer-events: none; }

/* Context-based state */
:host(:hover) { transform: scale(1.02); }
:host(:focus-within) { outline: 2px solid blue; }

::slotted Pseudo-Element

/* Style slotted children (one level deep) */
::slotted(*) { margin: 0.5rem; }
::slotted(h1) { font-size: 2rem; }
::slotted([slot="header"]) { font-weight: bold; }

:host-context (Limited Support)

/* Style based on ancestor */
:host-context(.dark-theme) {
    --bg: #1a1a2e;
}

---

⚠️ Important Notes

1. Shadow DOM required: css and styles only work with shadowDom enabled. Without shadow DOM, use regular page CSS.

2. CSSStyleSheet caching: Identical CSS text produces the same sheet instance. This is memory-efficient for shared base styles.

3. No <style> tags: Atomico uses adoptedStyleSheets exclusively. Avoid injecting <style> elements manually.

4. Specificity: Shadow DOM CSS is scoped. Styles don't leak out, and external styles don't leak in (except custom properties).