In Legalpower theme, sticky header(header that appears on top when you scroll the page) is in dark mode.
If you want to change it to light mode, there are 2 parts of the process. First lets change the background color to white. To do this, Please enter below css in Themeoptions/general/customcss box.
header.hdr.sticky-fixed { background: #FFFFFF; }
Feel free to change the color code in above code as it suits you.
Next part is to change the default menu item colors to dark, otherwise they will not be visible on white background as they are also white.
To do this, Please enter below css in Themeoptions/general/customcss box.
header.sticky-fixed .mainmenu .nav-holder > ul > li > a { color: #242424; }
Pasting both above css will make the sticky header like below image.