Unreadable desktop text with theme

This answer is only for the mentioned theme in the question, i.e "Windows-10-master" Theme.

in this particular theme, to change the text color of folders on Desktop.

i have saved my themes in /usr/share/themes/ you need to replace /usr/share/themes with ~/.themes in the below example as you saved your themes in ~/.themes/

I have created a nautilus.css file in the folder
/usr/share/themes/Windows-10-master/gtk-3.20/apps

made the content in this file as below

.nautilus-desktop.nautilus-canvas-item {
    color: blue; # <--------- here in the place of "blue" you can put any valid color you want
}

Save the file & Close.

now, Open the file gtk.css in the folder/usr/share/themes/Windows-10-master/gtk-3.20/

add the below line at the end,
@import url("apps/nautilus.css");

after adding this line, last 3 lines should look like this.

@import url("gtk-contained.css");
@import url("apps/gnome-terminal.css");
@import url("apps/nautilus.css");

Save the file & Close.

Reboot to see the change.

The issue here is with the text colour of the theme.

You can change this quite easily, since GTK+ themes (as of GTK+ 3) are simply CSS (Cascading Style Sheets).

You can change the colour to

#FFFFFF;

as opposed to

@desktop_item_fg;

in this section:

.caja-desktop,
.nemo-desktop.nemo-canvas-item,
.nautilus-desktop.nautilus-canvas-item {
    color: @desktop_item_fg;
    text-shadow: 1px 1px @desktop_item_text_shadow;
}

Or change the value of the variable:

@define-color fg_color

You might also want to consider a more up to date theme. You can find several of the same kind on gnome-look.org.