(style) add theme support

[cindrikj]

• replace all fixed values (colors, sizes etc) to sass variables
• create a default theme file with all variables and values
• add an option to change the size (default and small) of the:
  inputs, buttons, select, height of the rows, font size
• add an option to set the position of the pagination ( right is default position)
• add dark and light color scheme (based on primary colors)
• font-family should be also configurable