Improvements on the system theme

CHANGELOG.md

resources/styles/nheko-dark.qss

resources/styles/nheko.qss

resources/styles/system.qss

src/MainWindow.cpp

src/RoomInfoListItem.cpp

src/SideBarActions.cpp

src/TextInputWidget.cpp

src/TopRoomBar.cpp

src/TopRoomBar.h

src/UserInfoWidget.cpp

src/UserSettingsPage.cpp

src/Utils.cpp

src/Utils.h

src/dialogs/CreateRoom.cpp

src/dialogs/CreateRoom.h

src/dialogs/InviteUsers.cpp

src/dialogs/InviteUsers.h

src/dialogs/JoinRoom.cpp

src/dialogs/JoinRoom.h