PyQt5 - setStyleSheet 透明/alpha 不起作用？

Question

I'm currently writing some program using PyQt5, and I wanted to style the scrollbar in the UI

QScrollBar:horizontal {
    border: 0px;
    background-color: transparent;
    color: yellow;
    height: 15px;
    margin: 0px 20px 0px 20px;
}

QScrollBar::handle:horizontal {
    background: rgb(100,100,104);
    min-width: 20px;
}

QScrollBar::add-line:horizontal {
    border: 0px;
    background-color: transparent;
    color: yellow;
    width: 20px;
    subcontrol-position: right;
    subcontrol-origin: margin;
}

QScrollBar::sub-line:horizontal {
    border: 0px;
    background-color: transparent;
    color: yellow;
    subcontrol-position: left;
    subcontrol-origin: margin;
}

but it doesn't really show up when I run the program.

I also tried styling other items in the UI, this time by adding the stylesheets inline.

self.content = QTextEdit()
self.content.setStyleSheet("QTextEdit {border: 5px solid transparent}")

but this shows up:

Isn't that border supposed to be transparent? I've tried styling other items, but I just can't seem to make the transparent styling work. I've also tried using rgba(0,0,0,0) instead of transparent and I get the same results. Is there something I'm missing here?

EDIT: Here's the MRE as requested by @musicamante

import sys
from PyQt5.QtWidgets import *
from PyQt5.QtCore import *
from PyQt5.QtGui import *

class MainWindow(QWidget):
    def __init__(self, parent=None):
        super().__init__(parent)

        self.stylesheet_filename = "qss/styles.qss"
        self.loadStyleSheet(self.stylesheet_filename)

        self.setGeometry(200,200,800,600)

        self.layout = QVBoxLayout()
        self.layout.setContentsMargins(0,0,0,0)
        self.setLayout(self.layout)
        
        self.scene = MainScene()
        self.view = QGraphicsView()

        self.view.setScene(self.scene)
        self.layout.addWidget(self.view)

        self.show()

    def loadStyleSheet(self, filename):
        print('Style loading: ', filename)
        file = QFile(filename)
        file.open(QFile.ReadOnly | QFile.Text)
        stylesheet = file.readAll()
        QApplication.instance().setStyleSheet(str(stylesheet, encoding='utf-8'))

class MainScene(QGraphicsScene):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.setSceneRect(-1000,-1000,2000,2000)
        self.setBackgroundBrush(QColor("#252629"))
        self.card = Card()
        self.addItem(self.card)


class Card(QGraphicsItem):
    def __init__(self, parent = None):
        super().__init__(parent)    

        self.setFlag(QGraphicsItem.ItemIsSelectable)
        self.setFlag(QGraphicsItem.ItemIsMovable)  
        
        self.content = Content()

        self.grContent = QGraphicsProxyWidget(self)
        self.content.setGeometry(10,15, 280, 150)
        self.grContent.setWidget(self.content)

    def boundingRect(self):
        return QRectF(0,0,300,180).normalized()

    def paint(self, painter, QStyleOptionGraphicsItem, widget=None):
        #background
        path_background = QPainterPath()
        path_background.addRect(0,0,300,180)
        painter.setPen(Qt.NoPen)
        painter.setBrush(QBrush(QColor("#998090")))
        painter.drawPath(path_background.simplified())


class Content(QWidget):
    def __init__(self, parent=None):
        super().__init__(parent)

        self.layout = QVBoxLayout()
        self.layout.setContentsMargins(0,0,0,0)
        self.setLayout(self.layout)
        self.content = QTextEdit('I want the background of this QTextEdit and the scrollbar tracks to be transparent, not white.')
        self.content.setStyleSheet("QTextEdit {border: 0px; background-color: transparent}")
        
        self.layout.addWidget(self.content)
        

if __name__ == "__main__":
    app = QApplication(sys.argv)

    mainWindow = MainWindow()
    
    sys.exit(app.exec_())

EDIT 2: And I forgot to paste the qss code:

QScrollBar:horizontal {
    border: 0px;
    background: rgba(0,0,0,0);
    height: 15px;
    margin: 0px 20px 0 20px;
}
QScrollBar::handle:horizontal {
    background: red;
    min-width: 20px;
}
QScrollBar::add-line:horizontal {
    border: 0px;
    background: rgba(0,0,0,0);
    width: 20px;
    subcontrol-position: right;
    subcontrol-origin: margin;
}

QScrollBar::sub-line:horizontal {
    border: 0px;
    background: rgba(0,0,0,0);
    width: 20px;
    subcontrol-position: left;
    subcontrol-origin: margin;
}

Answer 1

While it might seem that the concepts of "standard" css can be applied to Qt StyleSheets (QSS), that is only true for the basic aspects of syntax and inheritance: QSS only implements some of the CSS 2.1 specification, and does not allow layout management (except for borders, margins and padding within the specific widget), which is exclusive responsibility of Qt.

This is very important when trying to set a "transparent" background for a scroll bar, because Qt doesn't allow the "overflow" effect of CSS, since the scrollbars are generally drawn outside the contents of the viewport.

In order to achieve a similar effect, the solution is to correctly set the background of the scroll area (the QGraphicsView, in your case).

QGraphicsView { 
    background: #252629; 
}

Note that, for various reasons, it's not always enough to set a generic stylesheet on the application, and it's sometimes required to explicitly set the scrollbar-dedicated stylesheet on the scroll bars ( self.view.horizontalScrollBar().setStyleSheet(...) , self.view.verticalScrollBar().setStyleSheet(...) ).

Then, the problem with the widget in the QGraphicsScene is different. When adding a QGraphicsProxyWidget, the widget is considered as a top level window, so it will have its own background, based on the application palette (which seems white, but it's a light gray). The solution, in this case, is the same as when trying to show a "transparent" window:

class Content(QWidget):
    def __init__(self, parent=None):
        super().__init__(parent)

        
        # ...