[Bug] CSS issues

[letsfindaway]

When experimenting with 1.8.0 I found some styling issues, probably related to changes in the CSS. I'm collecting these issues here as I discover them.

• Snap indicators have a square background, which should be transparent.
• Text on some buttons in nearly unreadable. E.g. the "Open" button in the file import dialog.
• In the same dialog, the toolbar is much higher than needed, resulting in strange button sizes.
• The indicators on a delegate frame, e.g. for rotating, are barely visible: white on light grey.