Filter icon variable name is inconsistent

[HeleenSG]

Describe the bug, issue or concern

Within the filter button the icon uses an ::after. But the variable used is "--filter-button-icon-before-open-content" and "--filter-button-icon-before-close-content" which is inconsistent.

Expected behavior

• Consistent naming
• Remove icon from before/after due to accessibility (Use HTML instead of CSS pseudo-elements for presentational icons #433)
• Maintain the option for styling to be set in a theme
• Add ability for theme to set icon position