How can I make icons in the Windows 10 taskbar/quicklaunch/tray darker to help prevent burn-in on my OLED display?

You can use the free AutoHotkey.

The following AutoHotkey script will cover the taskbar with a semi-transparent window that is click-through, but will hide it when a window becomes full-screen:

SysGet, Monitor, Monitor                   ; Get monitor dimensions
SysGet, WorkArea, MonitorWorkArea          ; Get monitor work-area without taskbar
dimtop := % WorkAreaBottom + 1             ; taskbar is assumed to start below the work-area
Gui Color, 0,0                             ; Black color
Gui -Caption +ToolWindow +E0x20            ; No title bar, No taskbar button, Transparent for clicks
Gui Show, X0 Y%dimtop% W%MonitorRight% H63 ; Create a semi-transparent cover window
WinGet ID, ID, A                           ; Get its HWND/handle ID
Winset AlwaysOnTop,ON,ahk_id %ID%          ; Keep it always on the top
WinSet Transparent,99,ahk_id %ID%          ; Transparency 99/256
SetTimer, coverIt, 500                     ; Repeat setting it to be on top of the taskbar
return

coverIt:
    WinGet style, Style, A                 ; Get active window style and dimensions
    WinGetPos ,,,winW,winH, A
    ; 0x800000 is WS_BORDER.
    ; 0x20000000 is WS_MINIMIZE.
    ; check no border and not minimized
    isfull := ((style & 0x20800000) = 0 and winH >= A_ScreenHeight and winW >= A_ScreenWidth)
    if (isfull) {
        WinHide, ahk_id %ID%
    } else {
        WinShow, ahk_id %ID%
        Winset AlwaysOnTop,ON,ahk_id %ID%      ; Ensure it is still on the top
    }
    return

After installing AutoHotKey, put the script in a .ahk file and double-click it to test. You may stop the script by right-click on the green H icon in the traybar and choosing Exit. To have it run on login, place it in the Startup group at C:\Users\USER-NAME\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup.