Current-line and selection shouldn't be the same

As dracula/dracula-theme#191 mentioned (but was closed due to inactivity), current-line and selection shouldn't be the same HEX value, as it confuses users. For example, if an editor supports the "select line" action, such as vim or other vi-like keybindings, it's hard to tell whether it's just the current line background or is the current line being selected.