Make equalizer leg design dependent on theme

### Enhancement Summary
To make the fader legs on the equalizer input and output the same as those on the fader legs of the FX-Mixer according to theme.

#### Justification
At the moment, the equaliser's fader appears to be completely independent of the theme, and is based on LMMS' default theme's fader legs regardless of the configuration of FX-Mixer legs according to theme.
It looks inconsistent, and it makes sense to change the design of the EQ-fader based on the theme, along with the FX-Mixer fader.

Start by locating the equalizer input and output fader implementation, then compare it with the FX-Mixer fader legs and their theme handling. Verify the result by viewing both faders under themes with different leg designs and confirm that the equalizer matches the FX-Mixer appearance.

Written by the indexing model from the issue text.