Get rid of inconsistently used $warning-color

[simon04]

The red $warning-color is used for various/four purposes. I propose to replace all usages by a more adequate color:

• Filter: submit button in contrast primary color
• SearchBar: change red
• ResultListItem: light primary color as background
• ResultListItem: transparent category color as bg

See also https://de.wikipedia.org/wiki/Wikipedia_Diskussion:WikiDaheim#Mein_Feedback_vom_2017-08-29

[reiterbene]

I would definetelly remove the check mark from the Fertig button - is moves the text to the left and lets the button look unaligned.