Guide · Agent skill
How to use the CSS Custom Properties Review agent skill
Updated · written with AI from the project's own documentation and checked against our catalogue data
The css-custom-properties skill is a set of written guidance that tells an AI agent how to review CSS for hard-coded colour, spacing and font-size values and move them into CSS custom properties (variables) on :root. It is for front-end developers cleaning up a stylesheet before a rebrand or dark mode, and for anyone learning how design tokens work in plain CSS. By the end of this guide you will know what the skill does, how to install it, how to ask for it in plain language, and where its limits are.
CSS Custom Properties Reviewthedaviddias/Front-End-Checklist · MIT Download the skill Quick start
- 1Download the ZIP. Get the css-custom-properties skill as a ZIP from the directory page.
- 2Unzip into a skills folder. Place the folder in ~/.claude/skills/ for personal use, or in a project's .claude/skills/ folder.
- 3Or upload it in the Claude apps. Open Settings, go to Capabilities, and upload the skill under Skills.
- 4Ask in plain language. Request a review or fix for a named stylesheet or component, mentioning colours, spacing or design tokens.
- 5Review the proposed changes. Check each suggested custom property and var() replacement against your design and naming before applying it.
What is an agent skill, and what does this one make the agent do?
An agent skill is a folder with a SKILL.md file that gives an AI agent instructions for a specific kind of task. When the skill is installed and relevant to your request, the agent follows its guidance.
This skill is drawn from the Front-End Checklist rule on CSS custom properties. When installed, the agent checks stylesheets for hard-coded colour, spacing and font-size values and suggests moving them into custom properties defined on :root, replacing repeated values with var() references. It can also explain how custom properties cascade and differ from preprocessor variables, and review responsive and interaction states for places where the rule is broken.
Install the css-custom-properties skill
Download the skill as a ZIP from the directory page and unzip it. The folder contains SKILL.md and references/rule.md.
For a personal install, unzip it into ~/.claude/skills/. For a single project, unzip it into the project's .claude/skills/ folder.
Alternatively, in the Claude apps, open Settings, go to Capabilities, and upload the skill under Skills.
- Personal: ~/.claude/skills/
- Project: .claude/skills/ inside your project
- Claude apps: Settings → Capabilities → Skills
Trigger the skill with natural requests
You do not need a special command. Describe the task in plain language that mentions stylesheets, colours, spacing or design tokens, and the agent can pick up the skill when it fits the request.
Good requests name the file or component and the goal, for example asking the agent to check a stylesheet for hard-coded values before a dark mode change.
- Check this CSS file for hard-coded colours, spacing and font sizes that should be custom properties.
- Extract the repeated values in this stylesheet into :root variables and replace them with var() references.
- Explain how custom properties cascade here and how they differ from Sass variables.
- Review this component's styles for breakpoint or hover-state places that break the token rule.
What to expect when the agent runs it
For a review, the agent points to exact selectors, declarations or breakpoints that hard-code values. For a fix, it proposes extracting repeated and meaningful values into custom properties on :root and replacing usages with var(). The source rule shows the style of token naming, for example grouping colours, a spacing scale, typography and radii.
For a fallback, the skill notes that var(--name, fallback) provides a default value when a property is not defined.
:root {
--color-primary: #2563eb;
--space-4: 1rem;
--text-base: 1rem;
}What the skill needs and what it does not do
The skill lists no requirements. It does not run scripts and does not use the network. It adds no executable tooling of its own; it is a checklist-style guide.
It reviews code, not rendered pages. Its description mentions checking the rendered layout, but no tooling in the files renders or screenshots pages, so expect code-level review only.
- Requires: nothing listed
- Runs scripts: no
- Uses network: no
Tips for better results
Give the agent the file or component, not just the topic. Ask for a before-and-after diff so you can check each replaced value.
Review suggested token names against your existing naming system before accepting them.
- Start with one stylesheet to see how the agent groups values.
- Check breakpoint and hover-state rules yourself, since the skill cannot see the rendered page.
What people use it for
- Tidying a CSS codebase by moving repeated colours and spacing into :root variables before a rebrand.
- Preparing a stylesheet for dark mode by centralising colour tokens.
- Reviewing component styles for hard-coded values that break consistency across breakpoints.
- Learning how custom properties cascade and differ from preprocessor variables.
Frequently asked questions
Is the css-custom-properties skill free?
The skill is published under the MIT licence. The source does not mention pricing, so check the directory page for any terms about how it is offered.
What do I need to install it?
The skill lists no requirements. Download the ZIP, unzip it into ~/.claude/skills/ or a project's .claude/skills/ folder, or upload it under Settings → Capabilities → Skills in the Claude apps.
Does the skill run code or access the internet?
No. The skill runs no scripts and uses no network. It consists of a SKILL.md file and a reference rule page.
Can the agent check how my page looks in the browser?
No. Despite its description mentioning rendered layout, the skill includes no tooling to render pages, so it reviews the CSS code only.
What is the difference between CSS custom properties and Sass variables?
The skill explains that custom properties cascade and inherit and are runtime values that JavaScript can read and write, while preprocessor variables are resolved at build time. Ask the agent to explain this with examples from your code.
Is this skill right for a project that already uses a design-token pipeline?
Probably not. The skill gives only general CSS guidance, so if your project needs tool-specific token output, pick a tool built for that pipeline.
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Our verdict
Use this skill if you are a front-end developer or learner who wants an agent to find hard-coded colour, spacing and font-size values and move them into CSS custom properties. Skip it if you already run a full design-token pipeline, want visual design output, or need the agent to check rendered pages, since it only reviews code.
Download the skill