Fully implement Spyder's new dark theme

Problem Description

This issue serves to track the sub-issues related to fully implementing the dark theme across Spyder’s various panes, text and icons, as a number of steps, both internal to Spyder’s codebase and external in QDarkStyle, are required.

Blocked by #8020 , the initial implementation. Originally spawned from #2350 , the initial issue and discussion thread.

Places that need to be “darkified”:

#8087: Dark css style for Help and Console panes
Dialogs:
- Pylint/Profiler Output window
- Dependencies dialog
- Github authentication
Interactive tour and menus:
- Interactive tour
- Online help and interactive tours submenus (of Help menu)
Help minor things
- Change color of math equations
- Change style of notes
- Theme scroll bar
- Theme table headers
Undocked windows
~~Non-inline plot dialog (not sure if possible)~~ @ccordoba12: This is not possible because that dialog is created by Matplotlib.
~~“Information” icon in file switcher isn’t themed~~ (opened as a separate issue to close this one out)

Places that still have black/dark colors:

Find in files results
Editor widgets:
- File switcher file list
- Symbol finder results
Outline Explorer code cell names
Preferences table columns:
- Keyboard shortcuts pref pane names column
- LSP Manager Command to Execute column
DataFrameEditor Index column
Pylint and Profiler panes’ date and rating text
Icons:
- “Gear”/pane options icon (all panes; dark gray is completely invisible/should be white)
- Regex icon (find in files, find/replace; black/should be white)
- About Spyder “i” icon (Help menu; black/should be white)
- Case sensitivity icon (find in files, find/replace; dark blue is low contrast/try light blue)

Other Spyder Issues/Bugs:

#8069 : Spyder dark theme overrides background color set in syntax highlighting theme, except behind text
#8070 : Explanatory text for Greedy Completion preferences option cut off only in dark theme
#8071 : Centralize remaining theme-related options (like Rstudio does) under a renamed “Themes” pref pane
#8072 : Consider making internal console respect user’s syntax highlighting theme
#8075 : Toolbar icons are too large particularly in dark theme
#8080 : Consider making the dark theme (and corresponding Spyder Dark syntax scheme) the default in Spyder 4
#8192 Fix options icon being very low contrast and sys.path and environment variables aren’t themed
#8284 : Improve Spyder-Dark syntax highlighting theme
Outlines around sections, fields, etc are invisible in tabbed preference panes
Fix SymPy equations being having black rather than white FG color with dark BG (new issue was opened about this).
#9439 : ~~Right code analysis sidebar does not match scroll bar position~~ (will be deal with separately)

External Issues:

ColinDuquesnoy/QDarkStyleSheet#111 : Text cut off and background color not full width when using comboboxes with a BG color
ColinDuquesnoy/QDarkStyleSheet#112 : Extremely difficult to read “disabled”/“grayed-out” text and embedded hyperlinks due to low contrast
ColinDuquesnoy/QDarkStyleSheet#118 : Tab close buttons for Editor and IPython Console are tiny and very difficult to see and click, and the hitboxes don’t match the icons. Tabs are very hard to tell apart from one another, especially when not the active tab or adjacent, and tabbar itself is hard to distinguish from the pane its attached to (and hard to tell which pane)
Fix toggle icons (regex, case sensitive) being impossible to tell whether they are activated or deactivated (better to be solved in QDarkstyle).