The @keyframes rule in CSS is used to create animations over a set duration. You define specific animation steps within the @keyframes rule, and then you can apply this animation to elements using the animation property. It allows you to control how an element changes over time.

The clear property in CSS is used to clear the effects of floating elements. It ensures that no element should touch the left or right-hand sides of a floating element and that it should appear below any preceding floating elements.

To use a custom property (also known as a CSS variable), you should use the syntax --property-name. Custom properties allow you to define reusable values in your CSS and use them throughout your stylesheet. They are preceded by two hyphens and are referenced with the var() function, like var(--property-name).

The process by which styles are applied to an element based on their source and specificity is known as the Cascade. Cascade refers to the process of determining which styles should be applied when there are conflicting style rules from different sources like user styles, author styles, and browser defaults.

The brightness filter in CSS is used to adjust the brightness of an element. It allows you to control how bright or dark an element appears, with values less than 1 making it darker, and values greater than 1 making it brighter.

To make text both italicized and bold, you should use the CSS properties font-weight: bold; to make it bold and font-style: italic; to make it italic. This combination creates text that is both thicker and slanted, resulting in bold and italicized text.

The CSS property used to change the text case, such as making text all uppercase, is text-transform. It allows you to control the capitalization of text, including options like uppercase, lowercase, and capitalize. This is especially useful for headings and other text elements where you want to change the case style.

The @keyframes rule in CSS is used to specify the keyframes for CSS animations. Keyframes describe how an animation progresses from the starting state to the ending state. They define the intermediate steps or percentage-based changes in the animation's properties, allowing for smooth and controlled animations.

For setting the "line-height" property to ensure consistent spacing, the recommended unit is typically "em." Using "em" units allows the line height to be relative to the font size, which ensures that the spacing remains consistent even if the text size changes. This is especially useful for creating responsive and accessible web designs.

To create a theme switcher in SASS/SCSS, you can define different color variables and use SASS/SCSS functions to return the appropriate color values based on the selected theme. This allows you to switch themes easily by changing a single variable and ensures consistency across your project.