Ensure we have sufficient color contrast

Part of #1106 (closed) (read this issue for access to the full audit and recommendations).

In particular:

• play buttons
• green buttons
• input fields borders

Minimum required value is 3:1, unless we have sufficient values for the dark theme.

Audit section (Section 1.4.5):

Some non-text elements have a low contrasting value. Some examples are: the orange Play-buttons (2.9:1; this colour combination also appears on page https://open.audio/library/playlists/186 ), the green buttons (f.i. ‘Login’, 2.6:1), the search icon (2.4:1) and the borders of the input fields (1.3:1; 2.1:1 when the input field receives focus). This should be at least 3:1. If the contrast switch in dark mode provides contrast ratios of at least 4.5:1, then it is not necessarily needed to change the contrast values for the light mode, but the dark mode doesn’t seem to work (yet).