Fix text colors for dark themes

[HebaruSan]

Problems

In a dark theme, the status log and parts of the mod info pane feature dark text on a dark background:

Looks fine in a normal theme:

Causes

1. A text box in .NET defaults to WindowText (black) on Window (white), but if you set its ReadOnly=true, as we do in this case, the background is changed to Control (dark gray) without changing the foreground, to give it a disabled appearance
2. We set the ForeColor of the mod info pane labels to ControlDarkDark; the meaning of this value is:
   Gets a Color structure that is the dark shadow color of a 3-D element.
   This color is not meant to be used for text foreground colors, and using it assumes a light background color.

Changes

Now these places look better.

1. The log text box's ForeColor is now ControlText, to match its background.
   
2. The labels in the mod info pane now have a ForeColor of GrayText, which the spec defines as "Gets a Color structure that is the color of dimmed text." This is meant to be used as a text color, and it will provide visual distinction from the normal text color:
   
   Standard themes still look OK:
   

Somewhat related to #2525 but not a fix for the specific things in that issue.

[Olympic1]

Looks good. I personally have trouble reading the links on the dark theme, could it be possibe to give it a lighter color using LinkColor?

I find System.Drawing.SystemColors.Highlight a good one.

[HebaruSan]

I'm not 100% sure Highlight is an improvement; at least in my theme, it's still pretty hard to read:

That's intended to be used as a background color for a selected element, and I'd rather not risk making things worse for some users by using it in a "clever" way. (The fact that SystemColors has no explicit LinkColor member says a lot about how long ago some of this stuff must have been designed!)