Suggestion: Fix gitgraph overflow

[Berk-Tosun]

Using gitgraphs in furo theme causes the figure to overflow and the sides cannot be viewed properly.

For me it can be fixed with the following configuration, "themeCSS" is where the magic happens:

%%{init: {
        'gitGraph': {
            'showBranches': true,
            'showCommitLabel': true,
            'mainBranchName': 'Ignore'
        },
        'themeVariables': {
            'commitLabelFontSize': '16px'
        },
        "themeCSS": ".mermaid svg {width: 100%;}"
}}%%

Perhaps someone can create a PR out of it, I will have a look at the source if I get the time.

Thanks for the nice package!