Widgets left not themeable

Based on a post made in #1839 I have made a list of the things that are not still themable at all or individually.

List of currently unthemeable items

• Title bars #2450
• Mixer background color #2704
• b+b editor background #2634
• Help text boxes
• Border styles
• Side menu styles

Themeable, but need improvement

• Qt Tooltips (Radius)

Screenshots:
Mixer background color #2704
b+b editor background #2634
QTooltip radius

General settings/Buffer size section

Buttons in general

Help textbox

Sidemenu

The sidebar gradient is vertical

Border style of some elements.

And plugins in general, not sure how viable this is, but skins for lmms plugins could be a good idea.

These are the ones I have found so far, I have removed some elements on my list because I decided they would be Fixes rather than editables. (Some elements that are not well centred, misused space, elements that are related in the .css file, etc...) As an example, the "sidebar gradient is vertical" could be considered something to fix because it is already editable, same with the QTooltip radius.

I can post the rest of the issues here or I can open another Issue asking for fixes and I can move the examples given there, whatever is better.

Start by reviewing the remaining unchecked items in the issue and the referenced issues #2704 and #2634, then locate the corresponding widget and theme code in the project. The scope should be narrowed to one item; done means that item can be themed individually and its behavior is verified in the relevant UI.