Added Gruvbox theme

[jagot]

I have added my favourite colour theme, Gruvbox!

These are just the colours, styling is up the user.

Building the documentation, the test suite fails somewhere else, I think unrelated to this PR.

[asinghvi17]

Hey, thanks for the PR! The color schemes look good and I will test more when back home.

Yeah, the test failure looks related to the new AlgebraOfGraphics release, so I'll probably have to go in and bugfix that.

[jagot]

I would like to have a bit more flexibility in choosing the colours, maybe through more keyword arguments, or allowing the user to change the colour cycle by passing in a subset of the symbolic names, but all of that can wait until a later version.

[jagot]

I added theming for some more elements, here is a simple test code that showcases some of it:

fig = Figure(size=(1280, 679))
Label(fig[0,:], "My fancy super-duper title", fontsize=25)

ax = PolarAxis(fig[1,1])

rs_h = IntervalSlider(fig[2, 1], range = LinRange(0, 1, 1000),
                      startvalues = (0.2, 0.8))
rs_v = IntervalSlider(fig[1, 2], range = LinRange(0, 1, 1000),
                      startvalues = (0.4, 0.9), horizontal = false)
                      
fig[3, 1] = buttongrid = GridLayout(tellwidth = false)

counts = Observable([1, 4, 3, 7, 2])

buttonlabels = [lift(x -> "Count: $(x[i])", counts) for i in 1:3]

buttons = buttongrid[1, 1:3] = [Button(fig, label = l) for l in buttonlabels]

fig[2,3] = Toggle(fig)

fig[2,4] = Menu(fig, options = ["viridis", "heat", "blues"], default = "blues")

fig[3,4] = Textbox(fig, placeholder="Hello there", width=300)

fig