Fix icon appearance

[gmikhail]

Updated toolbar icon:

• Text and underline centered
• Fixed underline size (to keep it from being cut off)
• Removed black background in dark theme icon to be transparent like the base icon. This fixes the ugly icon appearance with a custom toolbar color
• Fixed font for dark theme icon (now same as in base icon)
• Fixed script for converting to ico for dark theme icon, now transparency is handled correctly (works also with base icon, tested on Windows)

Comparison (left is old, right is new):

Please test how it works for you.

Initially, I only wanted to remove the unnecessary black background for the dark theme icon, but along the way I decided to fix the base icon appearance as well.

[Predelnik]

It does seem to work fine, sorry for not getting back more quickly