Exercise - Style your HTML with CSS


Cascading Style Sheets (CSS) let you specify how your page should look. The basic idea is to define what the style should be for the elements that you use within your HTML pages. While the HTML elements define your content, CSS styles define what this content looks like.

For example, you can apply rounded corners or give a gradient background to an element. Or you can use CSS to specify how hyperlinks look and respond when you interact with them. You can also perform sophisticated page layouts and animation effects.

You can apply styles to specific elements, all elements of a specific type, or use classes to style many different elements.

In this exercise, you'll apply CSS styles to HTML page elements and add some CSS code to define your light and dark themes. You'll also check the results in your browser's developer tools.

External CSS

In the previous unit about HTML, you linked to an external CSS file from HTML.

  <meta charset="utf-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Task Timeline</title>
  <link rel="stylesheet" href="main.css">

One benefit of external CSS is that multiple HTML pages can link to the same CSS file. If you make a change to the CSS, your styling will update for each page. Using an HTML file for your page content, a CSS file for styling, and a JavaScript file for interaction is called separation of concerns.

As described previously, you can also write CSS directly in HTML, which is called internal CSS. Even for a basic website, there are so many CSS rules the HTML page can become cluttered quickly. With more than one page, the same CSS would often be repeated and challenging to manage.

CSS rules

CSS rules are how you apply styles to HTML elements. CSS rules have a selector which is used to express which element, or elements, should the styles be applied to.

In Visual Studio Code, open the main.css file, and enter the following.

body {
    font-family: monospace;

ul {
    font-family: helvetica;

The above code snippet contains two rules. Each rule has:

  • A selector. body and ul are the selectors of the two rules and are used to select which element(s) the styles apply to.
  • An opening curly brace ({).
  • A list of style declarations that determine what the selected elements should look like.
  • A closing curly brace (}).

For example, the ul selector selects the <ul> HTML element in the page, to apply styles to it. The declaration is font-family: helvetica and determines what the style should be. The property name is font-family, and the value is helvetica.

As you'll see next, you can define your own custom names for elements.


ID and class selectors enable you to apply styles to custom attribute names in your HTML. An ID is used to style one element, whereas classes can be used to style multiple elements.

  1. Copy the following code into your CSS file, after the closing curly brace for ul selector that you added previously.

    li {
      list-style: circle;
    .list {
      list-style: square;
    #msg {
      font-family: monospace;

    The preceding code contains three CSS rules, with the last two rules using custom attributes to select elements: .list and #msg.

    • .list is a class selector. Each HTML element that contains a class attribute set to list will get the styles that are defined within this selector.

    • #msg is an ID selector. The HTML element that has its id attribute set to msg will get the styles that are defined within this selector.

    The names that you use for your selectors can be arbitrary, as long as they match what you've defined in the HTML.

  2. Save your work by selecting Control+S on Windows or Command+S on macOS.

View in browser

  1. To preview using VS Code, right-click the file name index.html, and then select Open In Default Browser.


    Even though you were just editing the main.css file, to preview the changes, you should select the index.html file.

    The webpage opens in your default browser.

    Screenshot of the website with the font styles applied.

Are the font styles what you expected to see? It's interesting how styles applied to the <body> are inherited on the <h1> element. We didn't define anything for <h1>, but it still got the font that was defined on <body>. This inheritance mechanism from parent elements to their descendants is one of the key aspects of CSS. However, the <li> elements have a different font, overriding the one set on <body> because they're also descendants of the <ul> element which you defined a style for.

Note that using Open In Default Browser in VS Code opens a new tab in the browser every time. To avoid opening a new tab, you can reload the tab that already contains your website instead.

To reload the tab press F5, which is the refresh keyboard shortcut, or press Ctrl+R on Windows or Linux, and Command+R on a Mac.

Add a light theme

Next, you'll add support for a color theme for your website. Begin by defining a light-colored theme using hex color codes.

  1. In your CSS file, add the following code at the end of the file.

    .light-theme {
      color: #000000;
      background: #00FF00;

    In this example, #000000 specifies black for the font color, and #00FF00 specifies green for the background color.

  2. In your HTML file, update the <body> element with a class name, light-theme, so the class selector for light theme will apply the styles correctly.

    <body class="light-theme">

View in browser

  • To preview using Visual Studio Code, right-click index.html, and then select Open In Default Browser or reload the previous tab by pressing F5.

    Notice that the light theme using a green background appears.

    Screenshot of the website with its light theme applied.

View applied CSS

  1. On the browser view, open Developer Tools.

    Right click the page and select Inspect, or select the shortcut F12 or Ctrl-Shift+I.

  2. Select the Elements tab and, inside the Elements tab, select the Styles tab (it should already be selected by default).

  3. Hover over the various HTML elements, and as you select a few elements, notice how the developer tools display which styles are applied to them in the Styles tab.

  4. Select the <body> element. Note the light-theme applied.

  5. Select the unordered list <ul> element. Note the custom style font-family: helvetica;, which overrides the style for the <body> element.

Screenshot of the website with its light theme applied and the Developer Tools next to it showing the Elements panel with the HTML and CSS code.

To learn more about viewing CSS styles in Developer Tools, check out the Get started viewing and changing CSS article.

Add a dark theme

For the dark theme, you'll set up the infrastructure in preparation for the next unit, in which you'll enable theme switching on the web page.

To add support for a dark theme to your CSS, use the following steps.

  1. Add some constants to the page root at the top of your CSS file.

    :root {
      --green: #00FF00;
      --white: #FFFFFF;
      --black: #000000;

    The :root selector represents the <html> element in the HTML page. For this kind of task, a best practice is to define a set of global CSS variables in a CSS rule with the :root selector. In this example, you've defined three color variables. You'll next be able to use these variables in other CSS rules.

  2. At the end of the CSS file, replace the light-theme rule with the following code to update it and to add the dark-theme selector.

    .light-theme {
      --bg: var(--green);
      --fontColor: var(--black);
    .dark-theme {
      --bg: var(--black);
      --fontColor: var(--green);

    In the preceding code, you defined two new variables, bg and fontColor, which specify a background and font color. These variables use the var keyword to set their property values to the variables previously specified in your :root selector.

  3. Next, in your CSS file, replace the current body selector with the following code.

    body {
      background: var(--bg);
      color: var(--fontColor);
      font-family: helvetica;

    In this example, you use the body selector to set the background and color properties and, because the elements that are visible on the web page are all inside the <body> element, they'll inherit the colors set on <body>.

  4. In your CSS file, remove the rules with the #msg and ul selectors so that they also inherit the same font from <body>.

  5. To view the dark theme, open the file index.html and manually edit the default theme in the <body> class attribute to dark theme (dark-theme), and then reload the page in the browser.

    Screenshot of the website with its dark theme applied and the Developer Tools next to it.

  6. Edit the <body> class attribute to switch the default back to light theme.

In the next unit, you'll use JavaScript to provide interactivity and support the switching of themes.