https://forum.unity.com/threads/clean-light-editor-theme.1127993/

light.uss

/*White*/
.AppToolbar, .ScrollViewAlt, .Toolbar, .dockArea, .ProjectBrowserBottomBarBg, .ProjectBrowserTopBarBg, .ProjectBrowserIconAreaBg, .TabWindowBackground, .PreToolbar, .IN-Footer, .IN-BigTitle, .SceneTopBarBg, .PreBackground, .RL-Header, .RL-Background, .RL-Footer, .hostview, .ScriptText {
    background-image: none;
    background-color: rgb(248, 250, 250);
    border-top-color: rgb(226, 228, 228);
    border-bottom-color: rgb(226, 228, 228);
    border-left-color: rgb(226, 228, 228);
    border-right-color: rgb(226, 228, 228);
}
 
/*Gray*/
.dockHeader, .FoldoutHeader {
    background-color: rgb(234, 236, 236);
    border-top-color: rgb(226, 228, 228);
    border-bottom-color: rgb(226, 228, 228);
    border-left-color: rgb(226, 228, 228);
    border-right-color: rgb(226, 228, 228);
}
 
/*WhiteBackground*/
.WhiteBackground, .TV-Selection {
    background-image: none;
    background-color: rgba(15, 15, 15, 0.1);
}
 
/*Interactables*/
.AppCommand,
.AppCommandLeft,
.AppCommandMid,
.AppCommandRight,
.AppToolbarButtonLeft,
.AppToolbarButtonMid,
.AppToolbarButtonRight,
.DropDown,
.DropDownButton,
.toolbarbutton,
.toolbarbuttonLeft,
.toolbarbuttonMid,
.toolbarbuttonRight,
.ToolbarDropDownToggle,
.ToolbarDropDownToggleLeft,
.ToolbarDropDownToggleMid,
.ToolbarDropDownToggleRight,
.ToolbarDropDown,
.ToolbarDropDownLeft,
.ToolbarDropDownMid,
.ToolbarDropDownRight,
.ToolbarPopup,
.ToolbarPopupLeft,
.ToolbarPopupMid,
.ToolbarPopupRight,
.TextField,
.MiniPopup,
.MiniPullDown,
.BoldTextField,
.toggle,
.Button,
.LargeButton,
.LargeButtonLeft,
.LargeButtonMid,
.LargeButtonRight,
.minibutton,
.minibuttonleft,
.minibuttonmid,
.minibuttonright,
.ButtonLeft,
.ButtonMid,
.ButtonRight,
.ObjectField,
.ObjectFieldButton,
.ColorField,
.AC-Button,
.IN-Title,
.ToolbarCreateAddNewDropDown,
.StatusBarIcon,
{
    background-color: rgb(248, 250, 250);
    border-top-color: rgb(196, 198, 198);
    border-bottom-color: rgb(196, 198, 198);
    border-left-color: rgb(196, 198, 198);
    border-right-color: rgb(196, 198, 198);
    color: black;
}
 
.AppCommand:hover, .AppCommand:checked,
.AppCommandLeft:hover, .AppCommandLeft:checked,
.AppCommandMid:hover, .AppCommandMid:checked,
.AppCommandRight:hover, .AppCommandRight:checked,
.AppToolbarButtonLeft:hover, .AppToolbarButtonLeft:checked,
.AppToolbarButtonMid:hover, .AppToolbarButtonMid:checked,
.AppToolbarButtonRight:hover, .AppToolbarButtonRight:checked,
.toolbarbutton:hover, .toolbarbutton:checked,
.toolbarbuttonLeft:hover, .toolbarbuttonLeft:checked,
.toolbarbuttonMid:hover, .toolbarbuttonMid:checked,
.toolbarbuttonRight:hover, .toolbarbuttonRight:checked,
.ToolbarDropDownToggle:hover, .ToolbarDropDownToggle:checked,
.ToolbarDropDownToggleLeft:hover, .ToolbarDropDownToggleLeft:checked,
.ToolbarDropDownToggleMid:hover, .ToolbarDropDownToggleMid:checked,
.ToolbarDropDownToggleRight:hover, .ToolbarDropDownToggleRight:checked,
.ToolbarDropDown:hover, .ToolbarDropDown:checked,
.ToolbarDropDownLeft:hover, .ToolbarDropDownLeft:checked,
.ToolbarDropDownMid:hover, .ToolbarDropDownMid:checked,
.ToolbarDropDownRight:hover, .ToolbarDropDownRight:checked,
.ToolbarPopup:hover, .ToolbarPopup:checked,
.ToolbarPopupLeft:hover, .ToolbarPopupLeft:checked,
.ToolbarPopupMid:hover, .ToolbarPopupMid:checked,
.ToolbarPopupRight:hover, .ToolbarPopupRight:checked,
.Button:hover, .Button:checked,
.LargeButton:hover, .LargeButton:checked,
.LargeButtonLeft:hover, .LargeButtonLeft:checked,
.LargeButtonMid:hover, .LargeButtonMid:checked,
.LargeButtonRight:hover, .LargeButtonRight:checked,
.toggle:hover, .toggle:checked,
.minibutton:hover, .minibutton:checked,
.minibuttonleft:hover, .minibuttonleft:checked,
.minibuttonmid:hover, .minibuttonmid:checked,
.minibuttonright:hover, .minibuttonright:checked,
.ButtonLeft:hover, .ButtonLeft:checked,
.ButtonMid:hover, .ButtonMid:checked,
.ButtonRight:hover, .ButtonRight:checked,
.ObjectFieldButton:hover, .ObjectFieldButton:checked,
.ColorField:hover, .ColorField:checked,
.AC-Button:hover, .AC-Button:checked,
.IN-Title:hover, .IN-Title:checked,
.ToolbarCreateAddNewDropDown:hover, .ToolbarCreateAddNewDropDown:checked,
.StatusBarIcon:hover, .StatusBarIcon:checked,
{
    background-color: rgb(228, 230, 230);
    border-top-color: rgb(196, 198, 198);
    border-bottom-color: rgb(196, 198, 198);
    border-left-color: rgb(196, 198, 198);
    border-right-color: rgb(196, 198, 198);
    background-image: none;
    color: black;
}
 
.AppCommand:active,
.AppCommandLeft:active,
.AppCommandMid:active,
.AppCommandRight:active,
.AppToolbarButtonLeft:active,
.AppToolbarButtonMid:active,
.AppToolbarButtonRight:active,
.toolbarbutton:active,
.toolbarbuttonLeft:active,
.toolbarbuttonMid:active,
.toolbarbuttonRight:active,
.ToolbarDropDownToggle:active,
.ToolbarDropDownToggleLeft:active,
.ToolbarDropDownToggleMid:active,
.ToolbarDropDownToggleRight:active,
.ToolbarDropDown:active,
.ToolbarDropDownLeft:active,
.ToolbarDropDownMid:active,
.ToolbarDropDownRight:active,
.ToolbarPopup:active,
.ToolbarPopupLeft:active,
.ToolbarPopupMid:active,
.ToolbarPopupRight:active,
.Button:active,
.LargeButton:active,
.LargeButtonLeft:active,
.LargeButtonMid:active,
.LargeButtonRight:active,
.toggle:active,
.minibutton:active,
.minibuttonleft:active,
.minibuttonmid:active,
.minibuttonright:active,
.ButtonLeft:active,
.ButtonMid:active,
.ButtonRight:active,
.ObjectFieldButton:active,
.ColorField:active,
.AC-Button:active,
.IN-Title:active,
.ToolbarCreateAddNewDropDown:active,
.StatusBarIcon:active,
{
    background-color: rgb(248, 250, 250);
    border-top-color: rgb(196, 198, 198);
    border-bottom-color: rgb(196, 198, 198);
    border-left-color: rgb(196, 198, 198);
    border-right-color: rgb(196, 198, 198);
    background-image: none;
    color: black;
}
 
/*Field*/
.TextField:hover, .TextField:active, .TextField:checked,
.MiniPopup:hover, .MiniPopup:active, .MiniPopup:checked,
.BoldTextField:hover, .BoldTextField:active, .BoldTextField:checked
.MiniPullDown:hover, .MiniPullDown:active, .MiniPullDown:checked
.DropDownButton:hover, .DropDownButton:active, .DropDownButton:checked
.DropDown:hover, .DropDown:active, .DropDown:checked
{
    background-color: rgb(248, 250, 250);
    border-top-color: #0F6ECC;
    border-bottom-color: #0F6ECC;
    border-left-color: #0F6ECC;
    border-right-color: #0F6ECC;
}
 
/*Tab*/
.dragtab-first, .dragtab {
    background-image: none;
    background-color: rgb(234, 236, 236);
    border-top-width: 1px;
    border-bottom-width: 1px;
    border-left-width: 1px;
    border-right-width: 1px;
    border-top-color: rgb(226, 228, 228);
    border-bottom-color: rgb(226, 228, 228);
    border-left-color: rgb(226, 228, 228);
    border-right-color: rgb(226, 228, 228);
    border-radius: 5px 5px 0px 0px;
}
 
.dragtab-first:hover, .dragtab-first:active, .dragtab-first:enabled, .dragtab-first:focus, .dragtab-first:checked,
.dragtab:hover, .dragtab:active, .dragtab:enabled, .dragtab:focus, .dragtab:checked {
    background-color: rgb(248, 250, 250);
}

https://forum.unity.com/threads/clean-light-editor-theme.1127993/
上記のフォーラムで紹介されていたスクリプト