Hi there -
well, it took some time to see your post anyway as I haven't online a lot recently
You actually can change/vary anything you like, as Hot Weather is totally skinable.
Easiest way:
Rightclick the desklet, choose 'Hot weather options' and click on the folder icon next to the skin's dropdown menu in the upper right corner.
Now the folder containing the present skin will be opened, showing the graphics and the text file ('skin.xml') containing the skin's font, font size, font color, coordinates of text, etc.
If unsure what to change, just make a copy of that folder, rename it and choose the new name as skin in the options (might need a restart or flip back to show up).
Then you e.g. can change the other days' info by changing
<Small_Icon_FontColor>clblack</Small_Icon_FontColor>
to
<Small_Icon_FontColor>clwhite</Small_Icon_FontColor>
If having problems with the high and low temp info, (strange bug as it works for me -might be due how big the font shows up on your system, maybe depending on font smoothing), change
<HighLowTemp_FontSize>18</HighLowTemp_FontSize>
to
<HighLowTemp_FontSize>16</HighLowTemp_FontSize>
Hope that helps...