Make equalizer leg design dependent on theme

[Monospace-V]

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.

[PhysSong]

Faders in EQ uses EqFader which is a subclass of Fader. Themes affect Fader, but not EqFader.

EqFader uses a different logic for peak calculation from Fader, that might be the reason there are two classes.

[Sea-Shore-Sahil]

Hi, I have a Software Methodology Development class that requires me to contribute to an open-source repo. Even though this issue does not have a good-first-issue tag, I would like to give it a try.

[Spekular]

@Sea-Shore-Sahil sorry about the late reply, but you're more than welcome to open a PR fixing this if you're still interested. The classes mentioned by PhysSong, the Fader property in data/themes/default/style.css, and the plugins/Eq folder may all be of interest.

https://github.com/LMMS/lmms/blob/master/plugins/Eq/EqControlsDialog.cpp

[Rossmaxx]

There's a small chance that @michaelgregorius might unify the EqFader with the Fader, from the discussion in #7045, which might fix this one.