For the Application Launcher (start menu) play with Blur and Background Contrast in Appearance under Desktop Effects in the System Settings (it’s $ kcmshell6 kcm_kwin_effects).
For the panel(s) (taskbar) you can use Panel Colorizer as suggested by schwarzerkater.
And before you say it I also agree (and posted twice) that at least a basic transparency adjusting slider should be baked-in by default 