ok thank you very much! found it.
if someone is interested:
you have to override
drawLabel (Graphics& g, Label& label)
this is how mine looks:
void OtherLookAndFeel::drawLabel (Graphics& g, Label& label)
{
g.fillAll (label.findColour (Label::backgroundColourId));
if (! label.isBeingEdited())
{
const float alpha = label.isEnabled() ? 1.0f : 0.5f;
Font font (getLabelFont (label));
font.setHeight (fontHeight);
g.setColour (label.findColour (Label::textColourId).withMultipliedAlpha (alpha));
g.setFont (font);
Rectangle<int> textArea (label.getBorderSize().subtractedFrom (label.getLocalBounds()));
g.drawFittedText (label.getText(), textArea, label.getJustificationType(),
jmax (1, (int) (textArea.getHeight() / font.getHeight())),
label.getMinimumHorizontalScale());
g.setColour (Colours::transparentBlack);
}
else if (label.isEnabled())
{
g.setColour (Colours::transparentBlack);
}
g.drawRect (label.getLocalBounds());
}
and in the custom l+f:
setColour (Slider::thumbColourId, Colours::transparentBlack); //
setColour (Slider::trackColourId, Colours::transparentBlack); //
setColour (Slider::backgroundColourId, Colours::transparentBlack); //