fix issue related to add plotly themes (addtheme functionality)

[gilbertogalvis]

This PR fix issues related to addtheme.

I share results bellow:

plotly_dark theme

x = randn(10000,1);
h = histogram(x);
f = fig2plotly(gcf, 'open', false);

addtheme(f, 'plotly_dark');
response = plotlyoffline(f);

web(response, '-browser');

ggplot2 theme

addtheme(f, 'ggplot2');
response = plotlyoffline(f);

web(response, '-browser');

seaborn theme

addtheme(f, 'seaborn');
response = plotlyoffline(f);

web(response, '-browser');

plotly_white theme

addtheme(f, 'plotly_white');
response = plotlyoffline(f);

web(response, '-browser');