Feature: Theme switcher

This update will introduce the new theme switcher feature.

Based upon the input of the SWT course I've created 2 Themes for arsnova.click (Dark and THM).

The owner of a session can choose the theme which shall be used for all attendees in the new created route /theme which is located between /answeroptions and /settimer

Changes

• All color definitions are removed from their respective .scss files. Instead they are now stored as variables in the new theme-switcher.scss file. When adding new elements which require a color (background, foreground, border, box-shadow, etc.) they must be added in theme-switcher.scss file aswell where the color is set to the variable
• All themes inherit the variables from the default arsnova.click theme but they can override them.
• Since CSS does not allow to cascade styles on keyframes all keyframes have to be added in the theme-switcher with all style settings they need.

ToDo's

• Implement "Elegant" theme (A theme for more sophisticated quizzes)
• Implement "ARSnova" theme (Like arsnova.eu)

Known issues

• Fix a few styling issues (regarding contrast, etc.) in the dark theme
• Fix a few styling issues (regarding contrast, etc.) in the THM theme
• Update the SVG logo file to be transparent
• Refactor the hljs classes (Check which of them are needed and think of good names for the SCSS variables, etc.)
• Add the theme to the session configuration when persisting it to the MongoDB (route from /timer to /lobby)

Preview

Theme "Dark":

Theme "THM":

Theme "Elegant":

Theme "ARSnova":