Improve colors

• Reduce contrast between background and text and adjust opacity according to this
• Make colors consistent
• Stop using grey raised buttons
• Use stroked or mat button for secondary action
• Stop using secondary color for buttons
• Make mat menu items consistent
• Fix some minor issues in dark theme

See some screenshots for compare:

Current colors (left) and slightly reduced contrast of background/surface and text (right)

Use of mat buttons for dialog abort action

Use of stroked button for secondary actions besides primary action

Use of primary color for icon buttons (more)

Consistent use of icon colors in menus (important=primary, critical=warn, other=grey)

Closes #588 (closed), #841 (closed)