Support config ventoy_left/ventoy_top/ventoy_color in theme.txt.

The configuration must be the first line in theme.txt and must be in the following format.
ventoy_left_top_color: "@5%@95%@#0000ff@"

The format is very strict:
1. ventoy_left_top_color must start with no space in front of.
2. left/top/color options must be around with 4 @