THEME - Custom fader - From three widgets to one theme... need help to adapt the one i already have

Community Usage questions
1

Hi

Using this theme, found somewhere here (and a little little bit readapted to work with a [0,16383] range fader) :

See THEME here 
.Fader_Bleu_Mix_Console {
    --knob-height: calc(0.186 * var(--height));
    --strip-padding: calc(var(--height) * 0.149);
    --fader-height: calc(var(--height) - var(--strip-padding) * 2);
    --value-adaptée: calc(var(--value) / 129);
}
.Fader_Bleu_Mix_Console canvas {
}
.Fader_Bleu_Mix_Console .fader_knob {
    position:absolute;
    left: 0;
    top: 0;
    right: 0;
    bottom: calc(1rem * (var(--strip-padding) - var(--knob-height) / 2));
    background-image: url('Images/fader_bleu.png');
    background-repeat: no-repeat;
    background-position: bottom;
    background-size: auto calc(1rem * var(--knob-height));
    pointer-events: none;
    transform: translateY(calc(
        -1rem * var(--value-adaptée) * var(--fader-height)
    ));
}

I manage to have this :
Fader rouge bleu

But now i'm trying to make some kind of adaptation in a compact mode :
I found the easy way ... One image widget / one fader. Both in a panel. Left and Top to zero, except for the image top set to #{390-3*@{fader.id}} (390 is a bit less than image and fader heights, and 3 depends on fader's size and range).
compactok

However... i'd really like to put all this in a theme, in order not to have three widgets here : panel, fader, image...

So, any clue to adapt this would be highly appreciated !

Thank you